We report a rare case highlighting the treatment strategy and literature review of synchronous, early-stage ipsilateral breast invasive lobular carcinoma (ILC) and marginal zone lymphoma (MZL). A 78-year-old woman presented with mammographically detected masses in the right breast. Diagnostic mammography revealed two distinct lesions: one at the 10 o'clock position measuring 1 cm, confirmed by biopsy as MZL, and another at the one o'clock position measuring 2.3 cm, diagnosed as estrogen receptor (ER)/progesterone receptor (PR) positive, HER2-negative ILC. Staging PET imaging revealed no evidence of locoregional or distant metastatic disease. The lymphoma was staged as stage IAE, and the ILC was staged as stage IA (pT1a, pN0, cM0, G1). A breast-conserving treatment approach was selected, consisting of lumpectomy and involved site radiation therapy (ISRT) to the right breast. Given the synchronous presentation of both malignancies within the same breast, whole breast irradiation was delivered using the UK Standardisation of Breast Radiotherapy (START) fractionation regimen (40.05 Gy in 15 fractions), allowing adequate dosing for both ILC and MZL. While standard dosing for breast-based MZL typically ranges from 24 to 30 Gy, the treatment plan was adjusted to address the synchronous ILC. At five-month follow-up, the patient showed no clinical or radiographic evidence of disease on surveillance mammography. She remained without clinical signs of MZL recurrence in accordance with the National Comprehensive Cancer Network (NCCN) guidelines. A review of the literature identified a case series involving 37 patients with synchronous breast carcinoma and non-Hodgkin lymphoma, among whom 5.4% had MZL and 10.8% had ILC. Only one patient in the series was reported to have both ILC and MZL, with the MZL staged as IV. Typically, the second malignancy is diagnosed after treatment of the first, resulting in sequential management. In contrast, our case is distinctive in that both cancers were discovered concurrently and at an early stage within the same breast, enabling simultaneous and coordinated treatment. This case suggests that in rare instances of synchronous early-stage breast cancer and lymphoma, a combined approach utilizing breast-conserving surgery and ISRT may offer an effective and streamlined treatment paradigm.