The team had less than a week's time to adjust to a new boss and offensive formation before setting down for Saturday's game at Livestrong Sporting Park, where Sporting Kansas City has one of the best home records in MLS this season.
All that proved too much for Toronto, which fell 2-0 and suffered its league-high 10th loss of the season.
C.J. Sapong and Julio Cesar scored goals for Sporting KC in the first half in front of a packed and boisterous crowd of 20,488.
"We worked on a lot of stuff this week," said Mariner, who took over for Aron Winter as Toronto FC manager earlier in the week and immediately shifted the team's offensive formation from a 4-3-3 to a 4-4-2. "I thought the team showed tremendous strength of character in the second half. The second half was excellent."
True, Toronto played the equal, or even better, than Sporting KC over the final 45 minutes. But by then, it had already spotted the home team two goals.
From the opening kickoff, Sporting KC imposed its will offensively and put the Toronto FC defence on its heels. Keeping especially busy was defender Doneil Henry, who was able to clear away several Sporting crosses into the Toronto penalty area early.
It was a clearance Henry missed, however, that ended with Sporting taking a lead it wouldn't relinquish.
Sporting KC newcomer Jacob Peterson, who joined the club in the off-season, fired a hard cross from the right side across the penalty area. Henry was waiting, but his attempt at getting a piece of the ball missed, and Sapong buried a shot past goalkeeper Milos Kocic for his fifth goal of the season and a 1-0 lead.
Toronto was without a shot until the 30th minute, when midfielder Eric Avila took a high-arching pass from Torsten Frings near midfield, got behind the Sporting defence and keeper Jimmy Nielsen. But Avila's shot toward an open net caromed off the left post straight across the goal mouth and off the right post before being cleared by KC defender Aurelien Collin.
It's just been that kind of season thus far for Toronto, 1-10-0 and in last in the Eastern Conference.
"I've hit one post, but never two," Avila said. "Seeing it hit one post, I thought it was in. It was heartbreaking. If it goes in, it's 1-1, and who knows what happens."
Five minutes later, Toronto's fortunes turned even worse when a corner kick from Sporting's Graham Zusi found the foot of Cesar at the far post. Kocic had no chance at the save as Cesar's bullet shot found the upper-right corner.
Toronto had its best scoring chance in the 70th minute when Luis Silva, a second-half sub, threaded a beautiful pass through to a breaking Danny Koevermans, who was looking for a goal for the third straight MLS game. But Nielsen got his left foot on Koevermans' shot, knocking the ball harmlessly over the endline.
"We came into a tremendous venue against a great team, give up two goals in the first half … . We could've put our heads down," Mariner said. "I'm very proud of our staff and very proud of the players. If we play like we did that second half, we're not going to lose many more games."