Nickel matrix composites reinforced by ceramic particles typically exhibit elevated strength but diminished toughness. In this paper, a novel in-situ micron-size TiC and nano-size γ′ phases reinforced Ni–Fe–Cr matrix composite was successfully prepared using the electron beam freeform fabrication (EBF3). Subsequently, the as-built composite was subjected to solution heat treatment at different temperatures. At a solution temperature of 1150 °C, the composite demonstrates superior strength-toughness synergy (ultimate tensile strength = 1109.4 ± 25.4 MPa, elongation = 12.08 ± 0.62%), exhibiting an increase of 17.1% and 104.4%, respectively, compared to the as-built composite. A systematic analysis of the strengthening and toughening mechanisms revealed that the γ′ phase played a predominant role in enhancing strength, while the size, morphology, and content of TiC phase significantly influence fracture toughness. After being solution heat treated at 1150 °C for 2 h, the γ′ phase precipitated uniformly with its size coarsening to 93.85 nm, demonstrating an optimal strengthening effect by shearing mechanism. Meanwhile, the TiC phase partially dissolved into the matrix and transformed from a long-strip shape to a more stable spherical shape. This transformation significantly reduced the formation of microvoids along the TiC/matrix interface during tensile loading, enhancing the fracture toughness. The γ′ and TiC phases were regulated by solution heat treatment to reach a balance of optimising properties, achieving synergistic enhancements in strength and toughness.