McClendon replaces Eric Wedge, who resigned at the end of the 2013 season after suffering a stroke in July.
"Lloyd is a bright, articulate guy," Mariners General Manager Jack Zduriencik said. "He is a tireless worker and very respected by the players with whom he has worked."
McClendon beat out a field of candidates for the Mariners job that included San Diego Padres bench coach Rick Renteria, Los Angeles Dodgers third base coach Tim Wallach, Chicago White Sox bench coach Joey Cora and Oakland Athletics bench coach Chip Hale.
He takes over a team that has failed to post a winning record since 2009, including 71-91 mark last season under Wedge, who missed 28 games due to his health problems.