Robertson is the primary guitarist for the band, playing dominantly rhythm guitar (evenly split between acoustic and electric) on nearly all of the band's songs. Two of his three bandmates also play guitar with the band on certain songs (Kevin Hearn sometimes on lead, and Jim Creeggan occasionally playing rhythm guitar for songs he sings lead on). During his tenure with the band, Steven Page also played rhythm guitar and, occasionally, lead guitar on around a third of the band's songs. Robertson 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. On each passing album, he has sung lead on an increased number of songs, to the point where he shared approximately equal duty with Page by the time the latter left the band. Robertson has sung lead on a number of the band's singles since the success of 1998's "One Week", the first single released with a Robertson lead (not including "If I Had $1000000" which featured a Page/Robertson call-and-response lead vocal). He, along with his other band mates, has taken over some of Page's lead and harmony vocal parts following the latter's departure. Robertson also sometimes includes rapping and, less often, beatboxing into performances with Barenaked Ladies.
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 an increased amount on his own. 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 does not typically compose, 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's band sings the theme song for The Big Bang Theory.