The 38-year-old, who departed Ibrox Stadium earlier this summer once his contract expired, succeeds David Hopkin as the Lions' new leader.
Miller took charge of his first training session on Saturday morning, with his first competitive match in charge of the West Lothian club coming against Airdrieonians in the Scottish League Cup on July 14.
Livingston director John Ward told his side's official website: "We're happy we've finally landed Kenny. We've been speaking since Monday, and it's not helped that his agent has been in Colombia with the time difference causing some issues.
"But it's a leap of faith for Kenny coming into management, and a leap of faith for the club too, but we're thrilled to have him here now and he'll be a big influence on the park as well as in the backroom."
Chairman Robert Wilson added: "I'm totally delighted Kenny has come after a long week. We interviewed him at the start of the week as John said, but it was only yesterday it was all finalised. His arrival can only help the club to move forwards now."
Miller has scored 200 goals in his senior career, taking in spells with nine different clubs.