His first notable guest role on television was in 1990 on the Quantum Leap episode "The Sea Bride – June 3, 1954". His first major television role was in the 1990s series Zorro as Alcalde Ignacio De Soto, the antagonist character who replaces Ramone as the Alcalde (Mayor) of Los Angeles in the third and fourth seasons. The show was filmed in a studio lot outside of Madrid, Spain, and was broadcast on The Family Channel. He was also involved in a television movie called Treasure Island: The Adventure Begins where he played a pirate called Black Dog. Treasure Island was broadcast to go along with a promotion for the casino resort in Las Vegas.

Hertzler began his acting career in the 1970s, doing mostly stage acting and appearing in some films. He guest starred in a few episodes for different television shows before landing the part of Alcalde Ignacio De Soto in the early 1990s show Zorro. In addition to Deep Space Nine, Hertzler has appeared on several other Star Trek shows, written two Star Trek novels, and has made appearances at Star Trek and science fiction conventions. Hertzler lives in the Finger Lakes region of New York where he was a lecturer at Cornell's theater department, and has been active in the area's regional politics, as well as writing a screenplay.

Hertzler was born in Savannah, Georgia. His parents were from Port Royal, Pennsylvania, and his family is descended from German-speaking Amish families. His father John G. Hertzler served in the U.S. Air Force, and his mother Eleanor Frances Beaver Hertzler was a Latin and French Teacher. His family lived in cities around the world including: St. Joe, Missouri; El Paso, Texas; and Casablanca in Morocco. He grew up mostly in the Washington, D.C. area, and attended Bucknell University, playing linebacker on their football squad. While there, he got into acting when the drama department recruited him for a production of Marat/Sade as they were looking for a big guy for one of the roles. After graduating in political science in 1972, he got his master's in set design at the University of Maryland, and attended law school for a year at American University. While he was in the DC area, he worked in the federal government, including with the Nixon Administration for the National Environmental Policy Act, and took an assortment of jobs to practice theatre including a waiter at a dinner theater, a bartender, and a cab driver.

Hertzler worked in the Washington area on theatre projects. On the movie screen, he portrayed Lucas in the horror film The Redeemer: Son of Satan (also known as Class Reunion Massacre), which was released in 1978. Hertzler also had a role in the feature film And Justice for All which starred Al Pacino, released in 1979. He also worked in New York City on the Broadway production The Bacchae as King Pentheus. In 1981, he moved to San Francisco to join the American Conservatory Theater (ACT) where he acted and directed in a number of productions including The Admirable Crichton, Richard II, Dial M for Murder, and a number of Shakespeare productions. He was an instructor with ACT, and had also worked with other theater productions such as Medea with the Cincinnati Playhouse. He moved to Southern California in 1988 and continued theatre work there.

In 1995–96, he was involved in a national tour for the stage theatre production of Dial M for Murder, in which he portrayed Max Halliday, an American Writer who gets involved in a love triangle.

During the 2005–2006 season, Hertzler played the role of Henry Drummond in productions of Inherit the Wind at Geva Theatre Center and Cornell University's Schwartz Center for the Performing Arts.

In 2007, Hertzler moved to Ulysses, New York, and became a Resident Professional Teaching Associate (RPTA) in the Cornell University Department of Theatre, Film & Dance, where he has directed or performed in a number of productions through the 2010 season. In 2009, Hertzler directed an outdoor production of A Midsummer Night's Dream for the Ithaca Shakespeare Company at the F.R. Newman Arboretum. He also portrayed Lord Capulet in Cornell's production of Romeo and Juliet, which involved students as well as professional actors.

In 2013, Hertzler ran for a position on the Ulysses town council His involvement in politics was partly because of the issue of hydraulic fracturing and concerns that the method of drilling for natural gas would destroy the Finger Lakes region. He also wanted the region's schools to get more state aid. He was elected and served on the board for a two-year term. In 2016, he was elected to the town council board for a four-year term extending to 2020.

Following the end of DS9, Hertzler played Hirogen in the Star Trek: Voyager episode "Tsunkatse", which was noted for featuring pro Wrestler Dwayne "The Rock" Johnson. In the episode, he and The Rock are conscripted alien fighters. Hertzler said that he also got to wrestle Actress Jeri Ryan who played Seven of Nine. In Star Trek: Enterprise, he played a Klingon Lawyer in the episode "Judgment" and a Klingon Captain in the episode "Borderland". He made appearances at Star Trek and science fiction conventions and events. In 2015, Hertzler joined the cast on the fan-funded short film Star Trek: Axanar as Captain Samuel Travis of the Federation.

Hertzler and fellow actor James Cromwell were among 19 people arrested for disorderly conduct during a protest outside a Crestwood Midstream compressor station in Watkins Glen, New York, on June 6, 2016. They were involved in the "We Are Seneca Lake" civil disobedience campaign against the Federal Energy Regulatory Commission green-lighting underground gas storage in Seneca Lake's salt caverns, despite public opposition.

On June 8, 2017, Hertzler announced his candidacy as a U.S. House Representative for New York's 23rd congressional district in the 2018 elections. As part of his campaign appearances, he plans to act "in the persona of Mark Twain", to present his ideas "through the brilliant humorist for all ages". He stated that this was a tribute both to Twain, who resided once in Elmira, and to actor Hal Holbrook, who had portrayed Twain on stage for more than six decades. On September 25, 2017 Hertzler announced his decision to leave the Democratic Party and run as an independent.