Robertson was the youngest of five children, and claims to have been named on a bet. He also claims to believe that his birth was an "accident"; due to his being several years younger than his other siblings, who were all born about one year apart. He credits the situation with allowing him to have the experience of being both in a big family, and an only child — once his siblings moved out of their home .

He was brought up in a home where country music almost exclusively played (he is still fond of it today). He would sing harmonies with his family and learned how to play guitar, which his father also played. Once he began playing guitar for other people, around the fifth grade, he knew he wanted to be a performer. He was in several bands during his school days including a cover band that played a notable collection of rock artists (Rush and Kim Mitchell being notable contributors to their repertoire).

In the fourth grade, Robertson entered the gifted program at Churchill Heights Public School. Robertson's first run-in with Page, who was a year ahead in the same gifted program, was when he unknowingly "stole" Page's best friend. It would be several years before the two would really speak to each other. Page saw Robertson at a Harvey's restaurant after a Peter Gabriel concert and was surprised to find that Robertson was also a fan. This ultimately led to them talking and becoming friends.

Robertson attended Woburn Collegiate Institute with Page, and around the twelfth grade, realized he needed to focus on passing his courses to get into university, believing his goal of being musician was somewhat of a "pipe dream". It was around this point when he began playing with Page. The two would become counsellors at Scarborough Schools Music Camp in the summer of 1988, and spent a lot of time together, singing songs and playing guitar. Page was flattered that Robertson knew some of his songs, which he had learned after acquiring a copy of a tape Page had made. Page was also impressed by Robertson's ear for singing harmony. Robertson ultimately turned to Page when he was scheduled to play at a benefit and his band had just broken up. The two played the benefit and in the process, formed Barenaked Ladies. He was accepted to York University, but dropped out after only half a year, choosing to devote his time completely to the band. He and Page spent the next two years building the reputation and following for Barenaked Ladies, joined along the way by Andy and Jim Creeggan, and Tyler Stewart.

Robertson is the primary guitarist for the band, playing dominantly rhythm guitar (evenly split between acoustic and electric). Three of his four bandmates also play guitar with the band on certain songs (Page sometimes on another rhythm part, Kevin Hearn sometimes on lead electric, and Jim Creeggan occasionally playing guitar for songs he sings lead on) but Robertson is the only member who plays guitar on nearly every song. He is capable of playing several other instruments including drums, pedal steel guitar, banjo and mandolin, but he rarely plays anything other than guitar, or occasionally 6-string banjo, on stage. He also considers himself a drummer (at some level at least). On each passing album, he has sung lead on an increased number of songs, to the point where he now shares approximately equal duty with Page, and has sung lead on a number of the band's singles since the success of 1998's One Week.

Robertson contributed to the writing for the band from their first album. Much of his contribution initially came in the way of co-writing with Page. As the years have passed, Robertson has written more and more on his own (and also more and more with Steve). He credits Jason Plumb in part for changing his perspective on songwriting, in that instead of trying to write one great song, he would commit to writing a certain number of ideas and they might not all be great, but parts of the songs would sometimes fit together into great songs. As well, Plumb encouraged Robertson to simply come up with an idea that fascinates him, and then to analyse it. The first song he wrote with this method was "When I Fall", based on the concept of "a window washer who's afraid of heights".

Besides Tyler Stewart, who doesn't typically write, Robertson is the only other band member who has not written and recorded songs outside of the Barenaked Ladies for himself in the form of a side project. Robertson has preferred to spend creative energy outside the band in co-writing and producing songs for other people (including Andy Kim and Jason Plumb).

Robertson was the second member of the band to get married, and the first to have children. He married his longtime girlfriend, Natalie, in June 1994. They had their first child, Hannah on November 17, 1995. Their second child, Lyle was born September 29, 1999. They also have a third child, Arden. As the first in the band to have children, he was also one of the first to advocate for more time at home with his family. When he is not working with Barenaked Ladies, Robertson prefers to spend as much time as he can with his family in Toronto.

Robertson lost a brother in a motorcycle accident, and the experience had an impact on him personally. It also came through in his songwriting; the song "Am I The Only One" was initially about his future wife, but ended up being about his brother's loss. The song "Leave" is about the image of his brother haunting him in hotel rooms while on tour.