It’s just impossible, what happened to me,” muses John Mahoney of his relatively late start in showbiz.

Born in England, Mahoney immigrated to the States as a teen, served in the Army, lost his British accent and spent years as an associate editor of a medical journal — all before joining the famed Chicago-based Steppenwolf Theater Co. and starring in dozens of distinguished onstage productions involving the likes of David Mamet and John Malkovich.

“To start acting at 37 and working in a profession where, at any given time, 95% of the people doing it are out of work — it’s just absolute luck,” Mahoney says.

Luck and, not to be underestimated, talent. The Tony Award-winning thesp (“The House of Blue Leaves”), perhaps best known for his Emmy-nominated turn as the crotchety Martin Crane on “Frasier,” stunned auds this season with his sympathetic portrayal of Walter, a felled Wall Street hero on “In Treatment.”

“He’s basically a very decent person going through something that we all hope we don’t,” remarks Mahoney of his character, who wrestles with recurring panic attacks as a result of his derailed pressure-cooker career and a host of unresolved familial issues.

“It’s almost Kafka-esque that this man who has gone by the book has been stabbed in the back and betrayed by those for whom he’s devoted his life through his work,” Mahoney says. “He constantly assumes responsibility for everything and wants to make everything right — even though certain things are not his fault. He finds himself in a strangely unexpected and undeserved situation.”

To prep for the emotionally taxing role, Mahoney opted not to base his interpretation of Walter on episodes from the original Israeli series, “Be tipul.”

“I watched one episode of that and I thought, ‘Oh, I shouldn’t watch this,’ ” says Mahoney. “I always do that, whether it’s a play or a movie or a part for TV. I want to make the character my own.”

What do you like most about your character?

“The thing I most admire is his loyalty. I love his faithfulness to his friends. There’s a great sense of loyalty and goodness to him — so much so that I sometimes just want to shake him and say, ‘Walter, for God’s sake, can’t you see what these people are doing to you?’ “