The 24-year old rejected more profitable overtures to remain at Tannadice Park, where he has been since he was a youth academy player in 2002, to pen a contract with the Saints.
And as some may seem moving from a club that finished fourth on the table to the eighth-placed outfit is a step down, Robertson certainly doesn't see it that way.
"I definitely don't see this as a step down in any way," he said.
"There wasn't much between the teams in games last season, but United had a good finish to the campaign. They wanted to keep me and there was talk of a testimonial - but I'm only 24!
"I thought long and hard about it, but I felt it was time for a change of scenery and this move feels right.
"Tony Docherty was my youth coach at United while the gaffer was my first captain. They were both outstanding and working with them again had a big part to play in my decision."