Singletary, a member of the Pro Football Hall of Fame thanks to his linebacker career with the Chicago Bears, will make his head coaching debut Sunday when the 49ers are home against Seattle.
"I'm very honored to be considered for this position, but getting there was a little difficult," Singletary said, referring to the fact he was taking his boss's job.
Singletary said he and Nolan had a long visit after the 49ers made the coaching move.
"He was emotional and so was I," Singletary said. "I felt like I came here for one reason, and that was to help Coach Nolan make this a winning organization.
"He (Nolan) said, 'I want you to understand something. I want you to take this job. I don't even want you to blink because I know you have the heart of the players at hand. I want you to take it to the next level.'"
San Francisco has lost four of its first five games this season. Singletary, 50, was in his fourth year as an assistant with the 49ers.