His management agency, Waring and McKenna, shared the news on Tuesday.
“It is with great sadness that we announce the passing of our wonderful client, Louis Mahoney,” the agency tweeted.
“Louis paved the way for many actors who followed: a lifelong activist and champion of anti-racism.
“His warmth and good humour will be sorely missed. A celebration of his life will follow.”
Mahoney was one of a few performers to appear both in the original run of Doctor Who as well as in the programme’s revival.
He starred in the 1970s episodes “Planet of Evil” and “Frontier in Space”, returning in 2007 as Old Billy in the episode “Blink”.
The actor also appeared in the 1975 Fawlty Towers episode “The Germans”. His other credits include the series Runaway Bay and Oscar Charlie, as well as the 2013 film Captain Phillips. Recently, Mahoney played in an episode of BBC One's The Split.
Born in The Gambia in 1938, Mahoney originally moved to the UK to study medicine, before pursuing an acting career instead.
Mahoney was a former vice president of the Equity trade union for performers, which honoured him on Tuesday with a message that read: “We are saddened to learn of the death of former Vice President Louis Mahoney. Louis was a passionate activist on behalf of @EquityUK and led the union’s fight against racism and apartheid for many years.”
Actor Peter Egan also shared a tribute to Mahoney, writing: “So sad to hear of the death of a good friend the wonderful actor #LouisMahoney an exceptional actor and active member of Equity and a well known resident of Hampstead. He will be missed... my condolences to his family.”
The Royal Court Theatre in London tweeted: “We’re so sad to hear of the passing of Louis Mahoney. A brilliant actor and the most wonderful human being; a devoted activist and extraordinary performer, he will be sorely missed.”