Hall, pictured, had been with Saffron for 10 years after spells with Central Trust and Aviva, rising to the position of chief executive three years ago.
Patrick Bunton, director of London and Country, said he was surprised to see the news but wished him "well for the future”.
Rob Jupp, chief executive of Brightstar, added: “It was a surprise to hear of Jon Hall’s resignation.
“We have enjoyed working with Jon and his senior management team over the last three years at Saffron
“With Jon’s leadership Saffron for Intermediaries has become one of the leading brands in the specialist mortgage market and I am certain Jon will reappear in another big role soon.”
AToM managing director Dale Jannels also lamented Hall’s resignation.
He said: “It’s a bit of a shock.
“He’s been there a while and he’s been very good for lenders, so he will be a loss.
“We’ve dealt with Saffron for the last few years, and it’s become a big player in the specialist manual underwriting sector.
“It’s good to have a human being you can have a good conversation with – and that’s what Saffron is.
“We always need those as we are always up against credit scoring lenders, whereas Saffron are manual. If there’s something that’s a bit complex you can have a good conversation with them.
“He will be missed.”
Saffron said an announcement will be made about his replacement in the coming weeks.
Geoffrey Dunn, Saffron’s non-executive chairman, said earlier: “Under Jon's tenure as CEO, the society has achieved substantial performance improvements and seen significant investment in new services for our members and improved facilities for our staff.
“Jon leaves the organisation in very good shape and the board, together with an extremely able executive team, will continue our focus on running a successful, solid and trusted society for our customers, our people and the local community.”