Jaklin Gulpashin, a woman visiting her mother at Humber River, said she saw the mayor leave via ambulance at the rear entrance of the building.
Nurses confirmed Ford was in one of the ambulances. Humber River Regional Hospital later confirmed the mayor would be heading to downtown Mount Sinai, on University Avenue's "hospital row."
There is still scant information about the mayor's overall condition.
Coun. Doug Ford, the mayor's brother and campaign manager, arrived at the hospital on Thursday morning for a visit. The usually talkative councillor did not say much to media, requesting both time and space before speaking to the press.
"[Rob]'s doing alright, he's keeping in good spirits," said Ford as he walked through the hospital parking lot to the entrance.
Renata Ford, the mayor's wife, was at the hospital on Thursday morning as well. She greeted Coun. Ford as he entered.
Neither family member was able to share news as to whether the tumour was benign or malignant.
When asked when he might comment on about what will happen next, the councillor said: "I need a day or so."
Coun. Ford said the news of his brother's hospitalization was "devastating" and said he didn't get any sleep last night.
The mayor's press secretary, Amin Massoudi, echoed what Coun. Ford said Wednesday night at the hospital.
"I just want to reiterate the request from the mayor's family for some privacy from the media at this time," Massoudi said from the mayor's office. "Again, that's very important — they've asked repeatedly for some privacy right now at this difficult time."
Massoudi said City Hall would continue with "business as usual."
Rob Ford, 45, was hospitalized Wednesday after seeking treatment for "unbearable" abdominal pain.
Doctors said the mayor will be kept in hospital over the next few days as tests are conducted to determine the kind of tumour.
Ford had abdominal surgery in 2009 to remove a tumour on his appendix. His father, Doug Ford Sr., died of colon cancer in 2006.
Ford is running for re-election in the city's Oct. 27 municipal vote and his brother wouldn't say how the tumour diagnosis might impact his campaign.
Massoudi, who has been with the Ford administration for most of his term, has suffered from cancer. He underwent surgery in March this year.
Massoudi went to visit Ford on Thursday afternoon.