Organisers insisted that the 22-year-old never planned to attend the red carpet launch at Somerset House with co-stars Sam Riley, Tom Sturridge and Harry Morgan.
However, the actress is rumoured to be "too ashamed" to make any public appearances in the wake of her cheating scandal.
Stewart admitted last month that she had been unfaithful to Robert Pattinson with Snow White and the Huntsman director Rupert Sanders.
Speaking at the premiere, Morgan said that it was "sad" Stewart couldn't attend, while Riley admitted he had "no idea" how she was doing following the incident.
"It's none of my business," Sky News quotes him as saying.
On The Road is the big screen adaptation of Jack Kerouac's 1957 Beat Generation novel about two friends who take a road trip across the US. It is due out in October.
Watch a trailer for On The Road below: