Ian Alexander Meldrum was born in Orbost, Victoria, on 29 January 1943. His father was Robert Meldrum (7 April 1907 – 1978), a farmer from Caniambo (25 kilometres (16 mi) from Shepparton) and then a World War 2 army sergeant – who served with the A.I.F. in Port Moresby – and his mother was Isobel Elizabeth (née Geer) (1912–1969) from Orbost. The couple married on 17 August 1940, two months after Robert's enlistment. Meldrum's younger brothers are Brian (born 1946, Mildura) and Robert (born 1950, Kerang).

Meldrum moved around during childhood and grew up largely with one of his grandmothers in Quambatook where he attended the local primary school alongside Future country music Artist, John Williamson. He also stayed with a number of aunts and was raised in the traditions of the Church of England. He developed a musical interest in Gilbert & Sullivan and Verdi. Meldrum's father later ran a hardware store in Kyabram. His mother had periodic hospitalisations for mental illness including some years at Larundel Mental Asylum, Bundoora in the mid-1960s. In the early 1960s Meldrum arrived in Melbourne where he briefly attended Taylors College. Initially intending to become a disc jockey he studied at a radio school. He would go to University of Melbourne – without formally enrolling – carrying law books, to eat lunch with the law students: "I hung around, I wouldn't even say I got into a course."

Kommotion was a teen-oriented daily TV pop music show, which had premiered in December 1964 on ATV-0, later Channel Ten. It included local performers miming to the latest overseas hits and artists showcasing their own material. In August 1966 its then-producer, David Joseph, was fired and most of the cast walked out in support. Al Maricic replaced Joseph and Meldrum reported the change-over for Go-Set. Maricic asked Meldrum to join the show: originally he declined but was convinced otherwise by Frazer, who reasoned that it would be good for their circulation. Episodes of Kommotion were directed by Rob Weekes.

The Groop had landed a recording deal with CBS Records. Meldrum followed them to Melbourne's Armstrong Studios, in late 1966, to observe the recording process. He learned production and engineering techniques from studio owner, Bill Armstrong, and in house engineer-producer, Roger Savage. Meldrum became involved with a number of artists' releases, including The Masters Apprentices' August 1967 single, "Living in a Child's Dream". Lead singer, Jim Keays, recalled that Meldrum "had quite an influence on the eventual outcome" as the unlisted assistant Engineer. He produced Somebody's Image's first three singles, "Heat Wave" (September), "Hush" (November) and "Hide and Seek" (April 1968). Their best performed single, "Hush", which peaked at No. 14 on the Go-Set National Top 40, was a cover version of Billy Joe Royal's track from earlier in 1967. Besides producing, he was also Somebody's Image's manager from early 1967 and formed a friendship with lead singer, Russell Morris.

From January 1968, Meldrum relocated to London, reporting in Go-Set on The Groop's efforts to break into the United Kingdom market; he also wrote about the English rock music scene. While there, Meldrum extended his networking to international contacts, including meeting Apple Records executive, Terry Doran, who introduced him to his idols, Paul McCartney and John Lennon. His writing style in Go-Set developed a camp form. Meldrum returned to Australia to attend his mother's funeral in May.

Despite some episodes of ineptitude, Meldrum became a major star in his own right and was a champion of local talent and regularly used the show to pressure radio stations to play more Australian music. McFarlane noted that alongside his bumbling, "Molly was a music fanatic, totally committed to, and passionate about, his work. Ultimately it was his drive that helped make Countdown so popular". As a result of his efforts, the show was able to make overnight hits of songs and performers it featured, and through the late 1970s and early 1980s it was a key factor in determining the direction of Australian popular music. By the mid-1980s its influence was waning, in part due to numerous other music video shows on commercial TV.

In 1974 Shrimpton and Weekes were meeting at the Botanical Hotel in South Yarra, formulating the concept for a new weekly TV pop music show aimed at the teenage market and decided they needed a talent scout; Meldrum walked in – to go to the bottle shop for a Scotch whiskey – and was given the job. The trio approached the Australian Broadcasting Corporation (ABC), with their idea based on the British show Top of the Pops and on Kommotion. Countdown premiered on 8 November, with Meldrum as the show's talent coordinator. He did not originally appear in the series, which had a different guest host each week.

Countdown was originally broadcast weekly, at 6:30pm on Friday evenings for 25 minutes. Contributing to its success was the move in January 1975 to a 6pm Sunday time-slot and extending to 60 minutes. Its reach was improved by a mid-afternoon Saturday time-slot to repeat the previous week's show. Countdown soon became the most successful and popular TV music program ever made in Australia, which exerted a dramatic influence on the local music scene over the next decade. The advent of colour TV in March 1975 coincided with a major shift in the direction of local popular music and was vital in the national success for artists such as Skyhooks and Sherbet. Countdown benefited from the emergence of the music video genre: it popularised promotional videos, which were previously a minor part of pop shows. Its use of film-clips, by both established and developing overseas acts (which rarely toured Australia), made Countdown an important venue for breaking new songs and new artists.

Meldrum produced the debut self-titled album for Supernaut in May 1976 and its related hit single, "I Like It Both Ways". He also promoted The Ferrets; he had them signed to Mushroom Records and started producing their debut album, Dreams of a Love, on 19 July 1976. After nearly a year, production was incomplete, so The Ferrets took over (assisted by audio Engineers, Tony Cohen and Ian MacKenzie) and finalised it on 15 August 1977 – Meldrum was attributed as Willie Everfinish. For its lead single he wanted the A-side as "Lies", taking weeks to produce it, and his preferred B-side, "Don't Fall in Love", was rushed in three hours. When The Ferrets premiered on Countdown, they used "Don't Fall in Love" instead, which reached No. 2 on the Australian Kent Music Report Singles Chart. Many customers wanted a copy of The Ferrets' album, however, there was concern at Mushroom as Meldrum had not yet organised its cover: a white, hand-stamped cardboard sleeve was issued with a promise of the artwork to follow.

In 1986, Shrimpton, Rule and Meldrum created another series, The Meldrum Tapes, for ABC with an international or local Artist interviewed in depth for 55 minutes — eventually 24 shows were made — which were later broadcast by MTV. Meldrum was noted for several on-screen gaffes, although the most "famous" of all was not originally broadcast. In a much retold incident, a clearly anxious Meldrum gushed during an interview on 13 November 1977, with Prince Charles, "I saw your mum in London in a carriage!" to which the Prince replied, "Are you referring to Her Majesty the Queen?" Although this incident is often related by Meldrum in interviews, it was not broadcast until later, as an out-take.

In August 1980 Gregg Flynn of The Australian Women's Weekly was on set during the taping of an episode which featured INXS, Doc Neeson (The Angels), Daryl Braithwaite (ex-Sherbet) and Toy Love. Flynn felt that Meldrum "appeared decidedly more healthy than some of his guest bands who looked as if anorexia nervosa was one of the side effects of guitar strumming." His appointment to the show had had "TV critics whipping themselves into a lather of hysterical accusations that the coiffured host was at best a cruel joke and at worst a danger ... [with his] mangled monologues as being detrimental to young people's vocabulary."

The following year, on 16 March 1981, Meldrum co-hosted the 1980 awards ceremony with international guests Suzi Quatro and Jermaine Jackson. Big winners were Cold Chisel with seven awards, which were not collected; the group performed the last live number, "My Turn to Cry", to close the show and then trashed their instruments and the set. The performance was seen as being directed at TV Week, Countdown, and Meldrum as being hangers-on. McFarlane felt the set trashing was a "protest against the show's vacuous nature". Sponsors TV Week withdrew their support for the awards and Countdown held its own ceremonies thereafter.

Meldrum has an adult adopted son, Morgan Scholes, who lives in China with his partner, Crystal Scholes, and the couple's child, Meldrum's grandchild. Meldrum's younger brother Brian is a former racing and golf Journalist and Editor. His youngest brother Robert is an actor, Director and Teacher. Although one of the first openly gay TV stars in Australia, he has said, "I had girlfriends. I was engaged a few times." In September 1976 his home in South Yarra was broken into; the thieves "stole sound equipment valued at $14,000." Shortly before 8pm on 11 October 1984, while Meldrum was in London to tape interviews with David Bowie, Boy George, and Billy Idol, a fire broke out in a hallway closet at his Richmond residence. The fire spread to the sitting-room, kitchen, and bedroom, with the 'Egyptian room' suffering moderate water and smoke damage. Meldrum's manager Ray Evans said that his personal record collection and an autographed photo of the Beatles were lucky to have survived the fire.

In February 1985, after Meldrum was announced as King of Moomba, he quipped "I was at the cricket the other day and the boys in Bay 13 at the Melbourne Cricket Ground were all yelling out 'Moomba' and 'hail the king'... not to mention a few 'hail the queen'". On 13 July Meldrum compèred the 1985 Oz for Africa concert — the Australian leg of the global Live Aid program running for four hours — which was broadcast in Australia on both the Seven Network and Nine Network and on MTV in the US. During December he used his industry contacts to organise a charity single for research on fairy penguins, he produced the recording of a cover of Lennon, Ono & Plastic Ono Band's "Happy Xmas (War Is Over)" by The Incredible Penguins with Angry Anderson (Rose Tattoo), Brian Canham (Pseudo Echo), Scott Carne (Kids in the Kitchen), John Farnham, Venetta Fields, Bob Geldof, Steve Gilpin (ex-Mi-Sex), Colin Hay (Men at Work), Hewett, Keays (ex-The Masters Apprentices), Brian Mannix (Uncanny X-Men), Wendy Stapleton (Wendy & the Rocketts) and Chris Stockley (ex-Axiom, The Dingoes).

On Australia Day (26 January) 1986, Meldrum was made a Member of the Order of Australia, with a citation for "service to the fostering of international relief and to youth". At the ARIA Music Awards of 1993, he received a Special Achievement Award to acknowledge his contributions to popular music. In 1994 at the Australasian Performing Right Association (APRA) Awards he obtained the Ted Albert Award (named in honour of Ted Albert). Music journalists, Toby Creswell and Samantha Chenoweth describe Meldrum as "The single most important person in the Australian pop industry for forty years" in their 2006 book, 1001 Australians You Should Know. In November 2014, he was inducted into the ARIA Hall of Fame, together with his TV show, Countdown, he became the first non-artist to receive the accolade.

The final episode of Countdown aired on 19 July 1987, followed by the 1986 Countdown Awards. Meldrum appeared at the end of the show wearing his Cowboy hat. He saluted the music industry and fans, then bared his shaved head in imitation of Midnight Oil's Peter Garrett and expressed regret that they had never appeared on the show. Dave Warner, musician and Writer, described Meldrum's impact "[he] was loved, loathed, reviled, respected, but above all, watched... You simply couldn't ignore [him] nor could the Australian music industry."

In March at the ARIA Music Awards of 1988 Meldrum was a presenter. A fracas developed between band manager, Gary Morris, accepting awards for Midnight Oil, and Meldrum. Morris felt that foreign artists such as Bryan Ferry should not present awards to local artists and made fun of Ferry's deliberately crumpled suit. Meldrum objected to Morris' disrespect to Ferry and he and Morris became embroiled. At the 1991 ceremony Morris provided a 20-minute acceptance speech on behalf of Midnight Oil: Meldrum disapproved of its length in the media. However, in 1993, when Meldrum received his ARIA Special Achievement Award for services to the music industry he provided one of the longest acceptance speeches in the ceremony's history.

In November 1998 Brian Mannix (ex-Uncanny X-Men) wrote and directed a stage play, Countdown: The Musical Comedy, with Meldrum portrayed by Michael Veitch. McFarlane observed "[it] was a loving and funny tribute to the Countdown era. It may have been shameless nostalgia, but with Veitch perfectly cast as Molly it was a hell of a lot of fun". It toured Australia through 1998 to 1999 and, in 2009, was revamped as Can't Believe It's not Countdown – It's a Musical Comedy. Meldrum also appeared on the tribute show, Countdown: Do Yourself a Favour, celebrating its 40th anniversary, which was broadcast by ABC in November 2014.

A televised roast, in 2003, for the openly gay Meldrum, Molly: Toasted and Roasted, was characterised by the recipient as a "gay bashing" due to its excessive homophobic slurs. Footy Show star Sam Newman received boos from the audience during his speech. Meldrum became a judge on 2004's Popstars Live, a reality talent quest program on Channel Seven, alongside fellow judges, Christine Anu and John Paul Young.

In September 2006, Meldrum's interview with Prince Charles on Countdown was listed at No. 41 in TV Week's 'Top 50 most memorable moments on Australian television' list. He made cameo appearances in Remembering Nigel (2007) and Ricky! the movie (2010). Meldrum is listed as co-author of Jenkin's 2007 book, Molly Meldrum presents 50 years of rock in Australia, where he provided comments on various Australian rock acts from 1958 to 2007. During September and October 2009, Meldrum appeared in Hey Hey Its Saturday reunion specials on the Nine Network despite working for the rival Seven Network.

Meldrum is a prominent supporter of the St Kilda Football Club in the Australian Football League (AFL) and the Melbourne Storm in the National Rugby League (NRL). Storm players continued their 2009 NRL Grand Final victory celebrations at his house in October that year. According to Google Books and Angus & Robertson, Meldrum co-wrote an autobiography, Some of My Best Friends Aren't: The Molly Meldrum Story with Journalist, Jeff Jenkins, in 2000, which was published by Random House Australia. However, The Age reported on 4 June 2007 that the book had still not appeared. In 2014 he finally published his autobiography, The Never, Um... Ever Ending Story: Life, Countdown and Everything in Between, co-written with Jeff Jenkins. This was followed up by a second book in 2016 titled Ah Well, Nobody's Perfect: The Untold Stories also co-written by Jenkins.

In early December, Meldrum interviewed UK singer and Britain's Got Talent runner-up, Susan Boyle. After signing with Seven to continue on Sunrise, Weekend Sunrise and Sunday Night he was unavailable for the 2010 season of Hey Hey It's Saturday. In February, Meldrum was appointed King of Moomba – his second appointment – with Kate Ceberano as Queen of Moomba. Since 2010 Meldrum has been a regular guest on Steve Vizard's daily radio show, commenting on sport, music, travel and current affairs.

On 15 December 2011, Meldrum was taken to the Alfred Hospital in a critical condition after being found unconscious in the backyard of his home in Richmond. He is believed to have fallen off a ladder from a height of around three metres. He was placed under intensive care in a sedated state and had surgery for his head injuries. As well as the head injuries, Meldrum had a broken shoulder, broken ribs, a punctured lung and cracked vertebrae. Meldrum had been with Steve Vizard on radio discussing the importance of health on the morning of the accident. By 27 December, further surgery to his chest injuries had occurred and his sedation levels were reduced. His brother said Meldrum had "spoken some words but they have no context".

On 8 January 2012, his brother Brian said Meldrum was breathing on his own and having conversations, but added his recovery would be slow. On 19 January Meldrum was taken out of hospital and moved into a rehabilitation centre. In April he gave his first public interview since the accident. A few months after the accident in 2012, Meldrum interviewed British pop singer Elton John and American pop singer Katy Perry.

Meldrum also produced several other hits—including Burns' top ten single, "Smiley", in December 1969—while continuing to write for Go-Set and a variety of magazines. Meldrum made his first of many visits to Egypt and by December had travelled on to UK, and through Doran, began working for Apple Corps as a publicist, which enabled him to score a scoop interview with Lennon and Yoko Ono, in which Lennon first revealed publicly that The Beatles were breaking up. Meldrum left UK in 1970 to travel to the US, reporting on the Los Angeles and New York music scenes and further establishing his contacts.

On 7 February 2016 the first part of a television series called 'Molly' aired on Australian television. It featured flashbacks of Molly's life and some actual footage of videos seen on Countdown. Molly was played by Samuel Johnson. The unaired footage from his interview with Prince Charles showed Johnson as Molly but actual footage of Prince Charles himself. Part two of the miniseries aired on 14 February 2016. The final scene was a very moving salute to Molly as he returns to public life after a lengthy hospital stay. Actual footage of Molly himself receiving a standing ovation as he walks out with the aid of a walking stick ends the series.