He had his breakout role in the 1968 RAI television series La freccia nera.

[1] After some further television success through the TV series Le terre di Sacramento and E le stelle stanno a guardare Merli made his film debut in the 1971 Miklós Jancsó's drama film La tecnica e il rito, then he became quite active in films of "poliziottesco" or political genre, often in leading roles.

[2] He is known for having dubbed over the voices of Clint Eastwood, Ed Harris, Robert Redford, Jack Nicholson, David Carradine, Brian Cox, Malcolm McDowell and Michael Caine in some of their films.

In his Italian dubbed animation roles, he voiced James P. Sullivan in Monsters, Inc., Mr.

[1][3] From Merli's relationship with the Swedish actress Eva Axén, he has one daughter, Euridice Axen, born in 1980.