• Ability to understand and the willingness to sign a written informed consent document and to comply with the study protocol procedures.

• Age ≥18 years at the time of signing informed consent. Because no dosing or adverse event data are currently available on the use of mosunetuzumab in patients \<18 years of age, they are excluded from this study.

• Histologically confirmed diagnosis of:

‣ follicular lymphoma (grade 1, 2, 3a, or not otherwise specified) or

⁃ marginal zone lymphoma (nodal, extranodal, or splenic), according to 2016 WHO classification and confirmed to express the CD20 antigen by immunohistochemistry or flow cytometry. Patients in whom definitive pathologic subtype of FL/MZL is undetermined due to limited biopsy material can be enrolled if in the investigator's opinion integrated clinicopathologic data are consistent with the eligible diagnosis.

• Agreement to provide, if available, lymphoma tissue for correlative analyses.

• At least one bi-dimensionally measurable nodal lesion, defined as \>1.5 cm in its longest dimension, or one bi-dimensionally measurable extranodal lesion, defined as \>1.0 cm in its longest diameter; with the exception of splenic MZL, which must be evaluable using the International SMZL Group criteria.

• No prior systemic therapy for B-cell lymphoma, except for palliative corticosteroids; prior local therapy (surgery, radiation therapy, or antibiotics) is allowed.

• Indication to start systemic therapy for lymphoma:

‣ Patients with FL must meet one of the GELF criteria:

• any mass ≥7 cm (except spleen);

∙ at least 3 nodes \>3 cm in diameter;

∙ symptomatic spleen enlargement;

∙ local symptoms or compromise of normal organ function due to tumor mass;

∙ presence of ascites or pleural effusion;

∙ presence of B symptoms (fever, night sweats, or unintentional weight loss of \>10% over ≤6 months);

∙ serum lactate dehydrogenase or beta-2-microglobulin above upper limit of normal;

∙ cytopenias due to underlying lymphoma (i.e., absolute neutrophil count \<1.0 × 109/L, hemoglobin \<10 g/dL, and/or platelet count \<100 × 109/L).

⁃ Patients with MZL must have an indication to start therapy as assessed by the investigator.

• Performance status ECOG 0, 1, or 2.

• Adequate hematologic function (unless due to underlying lymphoma as established by bone marrow involvement or splenomegaly):

‣ hemoglobin ≥9 g/dL,

⁃ absolute neutrophil count ≥1.0 x 109/L,

⁃ platelet count ≥75 x 109/L.

⁃ Glomerular filtration rate (GFR) ≥40 mL/min/1.73m2 using the Mayo Quadratic Formula.

⁃ The effects of mosunetuzumab on the developing human fetus are unknown. For this reason and because lenalidomide used in this trial is known to be teratogenic, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) and refrain from donating eggs or sperm throughout the treatment and for 3 months after the last dose of trial therapy. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately.

⁃ Agreement to enroll in and comply with all local requirements of the REVLIMID® (lenalidomide) Risk Evaluation and Mitigation Strategy (REMS®) program for the purpose of lenalidomide acquisition.