The 47-year-old was appointed as Jim Goodwin’s successor within three days of the Irishman’s departure for Aberdeen after Saints agreed compensation with Morecambe on Monday afternoon.
Robinson leaves the Shrimps sitting fourth from bottom of Sky Bet League One, after succeeding Derek Adams in the summer following their surprise promotion.
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Robinson, who stepped down as Motherwell boss in late 2020 after guiding the Steelmen to two cup finals and a third-place finish, said: “I think inevitably at some stage I was going to come back. I’ll be honest, I didn’t envisage it this quickly.
“If you had told me at one o’clock on Monday afternoon that I’d be taking a game at Dundee (on Wednesday), I wouldn’t have believed you.
“I didn’t search for the job, I didn’t apply for the job, I didn’t set out to leave Morecambe and it doesn’t sit well with me that I have left because you don’t set out to do that, you go down there to do the job.
“But St Mirren made a statement by what they did, they were prepared to pay compensation to bring me in and that gives you a lot of confidence that the people want to take the club forward.
“It was a tough decision but one I felt I had to make. I thought it was a real opportunity to continue to build on the good work already done, in the madness of Scottish football again.”
Robinson found out about St Mirren’s interest “completely out of the blue” on Monday afternoon and talks with vice-chairman Jim Gillespie persuaded him to sign a two-and-a-half-year deal.
“It is with a heavy heart that I leave because there are a lot of good people there and the people of the town were very good to me as well,” said Robinson, who has taken assistant manager Diarmuid O’Carroll with him from Morecambe.
“But when I spoke to Jim and he told me the ambitions, the set-up, the training facilities and what they are trying to improve with it, it was a very exciting opportunity that was difficult for me to turn down.
“The current squad was a bonus as well. Usually you come into a club and it’s a complete rebuild when a manager leaves.
“That’s certainly not the case here. I have inherited a lot of good players who are confident. I just need to tickle it along for now and gradually put my own stamp on it.”
Saints sit seventh in the cinch Premiership but will jump to fourth if they beat Dundee on Wednesday.
The game will reunite Robinson with the man he worked under and succeeded at Motherwell, Mark McGhee, who was a surprise appointment at Dens Park last week.
Robinson had got in touch then to congratulate his former mentor but will now be focused on spoiling McGhee’s first home match in charge.
“I texted him but he blanked me as usual,” the former Oldham manager laughed. “He never takes my text messages or calls.
“Mark was fantastic with me. You don’t get the amount of games Mark has got in football without having a brilliant knowledge and being a brilliant man-manager.
“I owe Mark a lot and I’m looking forward to seeing him, but I’m hoping to nick three points off him.”