Sully was ushered in and stood by the casket for a few moments before lying down while guests continued to pour in and pay their respects.
Earlier this week, an Instagram account for Sully posteda touching photo of the dog in front of Bush's casket. It featured the short caption, "Mission complete."
After he took in Sully in June, Bush showed him off on Twitter with a picture that included fellow former President Bill Clinton.
Sully will remain with the Bush family at least until the president is buried Thursday in Texas. He will be transferred to the Walter Reed National Military Medical Center's Facility Dog Program after the holidays.
On his Instagram page, Bush's son, former President George W. Bush, said it was an honor to have Sully and that he takes solace in knowing the dog will continue to help others.
"As much as our family is going to miss this dog, we're comforted to know he'll bring the same joy to his new home, Walter Reed, that he brought to 41," he wrote.