The 22-year-old came off the bench to score the winner in Tuesday's pre-season friendly against Doncaster.
"I tried to sign him for Crawley last year when he went to Yeovil and I tried getting him a couple of months ago," Evans told BBC Radio Sheffield.
The former Everton player scored six goals in 29 league appearances for the Glovers last season.
"Last season he scored nearly one goal in every two matches he started. He is hungry and is excited about the direction in which Rotherham is heading," Evans told the club website.
The former Arsenal academy product will go into the Millers' squad for their friendly against Hearts on Saturday.
Agard's goal against Rovers on Tuesday means Rotherham have now won all four of their pre-season fixtures so far this summer, including their first two games in their new home, the New York Stadium.
Rotherham have completed the signing of ex-Yeovil striker Kieran Agard, subject to him passing a medical.