Mr Mitchell was a colourful figure who represented the North East Lincolnshire port for almost four decades, from 1977 to 2015, after a successful career as TV presenter during the 1970s.
Following his death on Wednesday, Sir Keir said: “Austin served his constituency of Great Grimsby with remarkable commitment for 38 years.
“There are few MPs whose dedication to their constituents would translate into changing their surname to Haddock to promote local industry. His big sense of humour was matched by his deep Labour values. My thoughts are with his wife Linda and his children.”
Mr Mitchell was a well-known reporter and presenter with ITV’s Calendar regional news programme for many years before he won a by-election in Grimsby in 1977.
Many of those remembering Mr Mitchell recalled his most famous moment as a TV presenter when, in 1974, he chaired a prickly encounter between the former Leeds United managers Don Revie and Brian Clough.
Former Downing Street director of communications Alastair Campbell said on Twitter said Mr Mitchell had been “great company when I was with Labour, even when we disagreed”.
He added: “Best known of course as an MP, but a place in history as TV interviewer who did the ‘real life’ Brian Clough-Don Revie interview immortalised in Damned United. RIP.”
The Labour stalwart was born in Bradford in 1934 and studied history before moving to New Zealand, where he worked as a university lecturer in the subject in the 1960s.
His brief change of name to Austin Haddock came in 2002 as part of a bid to boost Grimsby’s fishing industry. Mr Mitchell, who leaves four children, married his second wife, Linda McDougall, in 1976.
Former Labour MP Melanie Onn, who succeeded Mr Mitchell in the Great Grimsby seat in 2015, said: “Austin will be greatly missed by many in Grimsby’s Labour family and across the town where he was taken from the TV screen into the hearts of local people, always ready with a camera and amusing anecdote.
“He was a larger-than-life character, secured lost pensions compensation for the last generation of Grimsby’s fishermen and was a vociferous opponent of council housing stock transfers to arms-length management organisations.”
She added: “On a personal level, Austin was unfailingly supportive to me as a new MP, still a bruising advocate of the Labour Party and gave his time, advice and expertise generously. It was a pleasure to get to know him and I send my heartfelt condolences to Linda, his family and close friends for their loss.”
Great Grimsby MP Lia Nici, who won the seat for the Conservatives in 2019, said she was sad to hear about the loss of “my friend and predecessor,” adding: “I learnt an awful lot from him. He worked incredibly hard for Great Grimsby and will be sadly missed.”