This randomized, multicenter, double-blind, placebo-controlled, parallel-group study will evaluate the efficacy, safety, and tolerability of lebrikizumab in adolescent participants with asthma whose disease remains uncontrolled despite daily treatment with inhaled corticosteroids (ICS) therapy and at least one second controller medication. Participants will be randomized in a 1:1:1 ratio to receive double-blind treatment with either lebrikizumab ('High' or 'Low') or placebo, administered as subcutaneous (SC) every 4 weeks (Q4W) for 52 weeks, in addition to their standard-of-care therapy. This will be followed by an optional 52-week double-blind active-treatment extension. The anticipated time on study treatment is up to 104 weeks. Participants who complete the study to Week 104, discontinue prematurely or decide not to take part in the optional active-treatment extension will transition to the 20-week safety follow-up period.