• Patients must have histologically confirmed World Health Organization (WHO) grade II or III meningioma that is progressive or with one or more recurrences following surgical resection and radiotherapy

• Patients must be eligible/appropriate for treatment with radiation therapy, specifically, if radiation is given in a single session, the target volume cannot exceed 8 ccs or if given in five (consecutive business days) sessions, the target volume cannot exceed 20 ccs

• Prior therapy:

‣ There is no limit on the number of prior surgeries or systemically administered therapeutic agents

⁃ An interval of \>= 28 days and full recovery (no ongoing safety issues) from surgical resection

⁃ An interval of \> 7 days from stereotactic biopsy

⁃ For prior systemic agents, participants must be at least 4 weeks (or 5 half-lives, whichever is shorter) from other prior cytotoxic chemotherapy (6 weeks from nitrosoureas) or biologic therapies

⁃ For prior radiotherapy, there are no exclusions on number of courses or prior type of radiotherapy. All modalities including prior fractionated external beam photon/proton radiotherapy, stereotactic radiosurgery, and/or or brachytherapy are permissible with a required interval of 6 months from last prior radiotherapy treatment, unless radiation given outside of the planned field, then within 2 weeks from prior radiotherapy treatment. Patients must have had one prior course of radiation therapy

• Participants must have recovered to grade =\< 1 or pretreatment baseline from clinically significant adverse events related to prior therapy (excluding alopecia, laboratory values listed per inclusion criteria and lymphopenia)

• Be \>= 18 years of age on day of signing informed consent

• Have a Karnofsky performance status (KPS) \>= 70

• Absolute neutrophil count (ANC) \>= 1500/microliter (uL) (within 28 days prior to enrollment)

• Platelets \>= 100000/uL (within 28 days prior to enrollment)

• Hemoglobin \>= 9.0 g/dL or \>= 5.6 mmol/L (within 28 days prior to enrollment)

• \* Criteria must be met without erythropoietin dependency and without packed red blood cell (pRBC) transfusion within last 2 weeks

• Creatinine =\< 1.5 x upper limit of normal (ULN) OR measured or calculated creatinine clearance \>= 30 mL/min for participant with creatinine levels \> 1.5 x institutional ULN (within 28 days prior to enrollment) (glomerular filtration rate (GFR) can also be used in place of creatinine or creatinine clearance (CrCl))

• \* Creatinine clearance (CrCl) should be calculated per institutional standard

• Total bilirubin =\< 1.5 x ULN OR direct bilirubin =\< ULN for participants with total bilirubin levels \> 1.5 x ULN (within 28 days prior to enrollment)

• Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase (SGOT)) and alanine aminotransferase (ALT) (serum glutamic pyruvic transaminase (SGPT)) =\< 2.5 x ULN (within 28 days prior to enrollment)

• International normalized ratio (INR) OR prothrombin time (PT) =\< 1.5 x ULN unless participant is receiving anticoagulant therapy as long as PT or activated partial thromboplastin time (aPTT) is within therapeutic range of intended use of anticoagulants (within 28 days prior to enrollment)

• Activated partial thromboplastin time (aPTT) =\< 1.5 x ULN unless participant is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants (within 28 days prior to enrollment)

• Additional required screening labs within 28 days: albumin, Alkaline phosphatase, calcium, total bilirubin, blood urea nitrogen (BUN), creatinine, total protein, random glucose, potassium, sodium, chloride,bicarbonate, and magnesium, phosphorus.

• Magnetic resonance imaging (MRI) within 28 days prior to start of study drug. Corticosteroid dose must be less than or equal to 2 mg and at a stable or decreasing for at least 5 days prior to the scan. If steroids are added or the steroid dose is increased between the date of the screening MRI and the start of treatment, a new baseline MRI is required

• Ability to understand and the willingness to comply with scheduled visits, treatment schedule, laboratory testing, and other requirements of the study, including disease assessment by MRI, as confirmed by signing a written informed consent document

• The effects of pembrolizumab on the developing human fetus are unknown. For this reason:

‣ Male participants:

‣ \*\* A male participant must agree to use a contraception during the treatment period and for 120 days after the last dose of study treatment and refrain from donating sperm during this period

⁃ Female participants:

∙ A female participant is eligible to participate if she is not pregnant, not breastfeeding, and at least one of the following conditions applies:

⁃ Not a woman of childbearing potential (WOCBP) OR

• A WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 120 days after the last dose of study treatment