For the most part, Hayward has used a red Gibson ES-335, though he also uses other guitars in both performing and recording, including a 1955 Martin D-28 "Dreadnought", a James Olson six-string acoustic, a black Guild acoustic, a Squier Stratocaster (essentially an inexpensive Fender Stratocaster, as Squier is a subsidiary of Fender), a Fender Telecaster, a blonde Guild 12-string acoustic (tuned to "open C" for "Question"), and in 1967 a black Gibson Les Paul. Between 1965 and 1968, he was without his Gibson 335 and relied on other instruments, most notably a 1964 Fender Telecaster and a hand-built 12-string guitar he had renovated for Donegan (he eventually bought this guitar from Donegan's widow). However, in an interview included on the Lovely to See You concert DVD (2005), Hayward says the 1963 Gibson 335 has been with him since 1967. Recently, he has played a Collings D3 on stage and on recordings. Among other instruments, Hayward also played mandolin on A Question of Balance and sitar on In Search of the Lost Chord.

After beginning by singing the old blues-inspired repertoire of the Moody Blues' 1964–1965 era, Hayward's initial artistic contribution to the Moody Blues was his song "Fly Me High", which was a Decca single early in 1967. It failed to chart, but gave the revised band a new direction forward from the R&B sound they had been largely producing up to that point.

In 1966, after answering another ad in Melody Maker, this time placed by Eric Burdon of The Animals, Hayward was contacted by Mike Pinder of the Moody Blues after Burdon had passed on Hayward's letter and demo discs to Pinder. Within a few days, Hayward had replaced departing Moody Blues vocalist and Guitarist Denny Laine. Bassist John Lodge replaced temporary deputy Rod Clarke, who had stood in for departed Bassist Clint Warwick at the same time.

The Moody Blues' attempts to come up with another hit single during 1967–68 had them record three other Hayward compositions; "Long Summer Days", "King and Queen", and "What Am I Doing Here?", all of which were then left unissued, but together with unissued songs by Pinder and Lodge they later formed the 'studio side four' of Decca's 1977 release Caught Live Plus Five which largely comprised a December 1969 live recording of a concert at the Royal Albert Hall (issued against the group's wishes).

Written at the end of one love affair and the beginning of another, the song "Nights in White Satin" was, according to Hayward, "in adoration of all women". Hayward married model Ann Marie Guirron on 19 December 1970. His daughter Doremi, who sings on the track "Raised on Love" on Hayward's 1977 album Songwriter, was born on 3 December 1972.

In 1974, the Moody Blues decided to take what ended up being a four-year break from performing and recording. Hayward continued working with Lodge and Producer Tony Clarke, using Musicians from the Moody Blues' label, Threshold, and sounding very much like the mother group. Together, they had a hit in 1975 with "Blue Guitar" (a Hayward recording with the band 10cc) and released an album titled Blue Jays.

In 1977, Hayward recorded his first solo album Songwriter. He enjoyed international solo success in 1978 when he appeared on Jeff Wayne's Musical Version of The War of the Worlds concept album, which yielded his hits "Forever Autumn" and "The Eve of the War". Wayne later contributed to Hayward's 1980 album Night FLIGHT.

Their album sales from 1978 to the present total more than 60 million. This is the regularly quoted estimate of their album sales, since the total sales of their albums before 1978 are disputed owing to lack of official record company data, However, the period 1967 to 1974 was when their albums (and singles) were charting highest in the UK and US plus worldwide (album track "Melancholy Man" made number one in France as a single in 1970) - Days of Future Passed topped the US album charts on reissue in 1972, then was followed into the album charts by the new studio album Seventh Sojourn.

During the 1980s, Hayward composed and performed for film and television, including the theme song "It Won't Be Easy" for the 1987 BBC2 science-fiction series Star Cops, "Something Evil, Something Dangerous" for the film Howling IV: The Original Nightmare, "Eternal Woman" for the film She, and music for the animated television series The Shoe People.

Hayward's songs have opened each of the Moody Blues albums in their 'post-Mike Pinder era' since Long Distance Voyager in 1981, and his songs, both solo compositions and co-written with Lodge, plus his lead vocals, harmony voice, and guitar playing, have been a major overriding factor in the band's work and continued success since 1981.

In 1989, with producer-arranger Mike Batt, Hayward released Classic Blue, an album of pop standards written by other composers set to orchestration arranged by Batt. Classic Blue included a cover version of Led Zeppelin's hit "Stairway to Heaven". Hayward's solo album The View from the Hill was released in 1996, and a live recording, Live in San Juan Capistrano, followed in 1998.

On 10 March 1997, Hayward was featured on the BBC One's programme This Is Your Life with Michael Aspel.

Hayward contributed vocals to a song on Rick Wakeman's 1999 album Return to the Centre of the Earth.

In June 2003, he gave several performances at the Rock and Roll Hall of Fame. Later that year, he sang along with other rock Singers on another orchestral album, consisting of Moody Blues songs with the Frankfurt Rock Orchestra, titled Justin Hayward and Friends Perform the Hits of the Moody Blues (alternatively called Justin Hayward and Friends Sing the Moody Blues Classic Hits). Hayward was later involved in a legal dispute, since resolved, arguing he was not paid for his participation on the album.

The Moody Blues, with Hayward, Lodge, and original Drummer Graeme Edge, continue to tour extensively, and in a BBC World Service interview, Hayward and Lodge made it clear they have no plans to stop working, regarding it as "a privilege" to still be working in the music industry. In an interview in 2005, Edge said if he remained in good health, he could go on for 10 more years.

In April 2006, Hayward took part in the stage tour of Jeff Wayne's Musical Version of The War of the Worlds, reprising his role in autumn 2007 in Australia and in the UK in December 2007. He did so again in the UK in June 2009 and appeared on the tour in November and December 2010.

On 10 December 2011, Hayward, along with Ian Anderson from Jethro Tull and Bruce Dickinson from Iron Maiden, played a concert together at Canterbury Cathedral.

Hayward was awarded the first of numerous awards from the American Society of Composers, Authors and Publishers (ASCAP) for songwriting in 1974. In 1985, the Moody Blues picked up the Ivor Novello Award for Outstanding Contribution to Music, and in 1988, Hayward received the Novello, among other honours, for Composer of the Year (for "I Know You're Out There Somewhere"). In 2000, he was one of a handful of British artists to receive the "Golden Note" award for lifetime achievement by ASCAP. In 2004, Hayward was awarded the "Gold Badge" for lifeongwriters, Composers and Authors. At the Ivor Novello Awards in London on 16 May 2013 Hayward was given the PRS for Music Award for Outstanding Achievement (presented to Hayward by Marty Wilde).

Hayward wrote the band's UK number two hit "Question" as well as "Voices in the Sky", "Driftwood", "The Voice", "Blue World", "Your Wildest Dreams", "I Know You're Out There Somewhere", "English Sunset", and "December Snow"—in all, writing 20 of the group's 27 post-1967 singles.

In April 2015, Hayward was interviewed on BBC Radio 2's Johnny Walker show.

The Spring of 2016 saw another Hayward release from producer/director David Minasian. Titled Live in Concert at the Capitol Theatre, the DVD contained a few surprises including the first live performance of "You Can Never Go Home" from the 1971 Every Good Boy Deserves Favour LP. Also featured was a bonus studio recording of a new song titled "The Wind of Heaven", a joint composition between Minasian and Hayward. Intended as the main theme for a forthcoming motion picture, the song, with an elaborate music video directed by Minasian, was released ahead of schedule on the DVD to coincide with Hayward's 2016 US solo tour.