After school, Finch took several badly paid jobs until he started acting in small parts for Doris Fitton in 1934. He worked as a sideshow spruiker at the Royal Easter Show, in vaudeville with Joe Cody and as a foil to American comedian Bert le Blanc. A recommendation led to work with George Sorlie's travelling troupe, which in turn led to the attention of Australian Broadcasting Commission radio drama producer Lawrence H Cecil, who was to act as his coach and mentor throughout 1939 and 1940. He was "Chris" in the Children's Session and the first Muddle-Headed Wombat. He later starred with Neva Carr Glyn in an enormously popular series by Max Afford as husband-and-wife detectives Jeffery and Elizabeth Blackburn as well as other ABC radio plays. He landed his first film in 1938, Dad and Dave Come to Town, a small part which failed to attract any notice.
At the time of his death, he was doing a promotional tour for the 1976 film Network in which he played the television anchorman Howard Beale who develops messianic pretensions. He was posthumously nominated for a Best Actor Oscar for that role, winning the award, which was accepted by his widow, Eletha Finch. Although James Dean, Spencer Tracy, and Massimo Troisi were also posthumously nominated for a Best Actor Oscar, Peter Finch was the first actor to have won the award posthumously, as well as the first Australian actor to win a Best Actor award. He was the only posthumous winner of an Oscar in an acting category until Heath Ledger won the Academy Award for Best Supporting Actor in 2009 (there were many earlier posthumous Oscar winners in non-acting categories; Ledger was also an Australian). Finch also won five Best Actor awards from the British Academy of Film and Television Arts (BAFTA), including one for Network.
He had four children from his three marriages: Samantha, Charles and Diana with Yolande Turner, and Anita with Tamara Tchinarova.
Hollywood Forever Cemetery in Hollywood, Los Angeles, California.