The relationship between microstructural features and macroscopic mechanical properties of engineered tissues was investigated in pure and mixed composite scaffolds consisting of collagen Type I and fibrin proteins containing embedded smooth muscle cells. In order to vary the matrix microstructure, fibrin polymerization in mixed constructs was initiated using either the blood-derived enzyme thrombin or the snake venom-derived enzyme ancrod, each at low and high concentrations. Microstructural features of the matrix were quantified by analysis of high resolution scanning electron micrographs. Mechanical properties of the scaffolds were assessed by uniaxial tensile testing as well as creep testing. Viscoelastic parameters were determined by fitting creep data to Burger’s four-parameter model. Oscillatory dynamic mechanical testing was used to determine the storage modulus, loss modulus, and phase shift of each matrix type. Mixed composite scaffolds exhibited improved tensile stiffness and strength, relative to pure collagen matrices, as well as decreased deformation and slower relaxation in creep tests. Storage and loss moduli were increased in mixed composites compared with pure collagen, while phase shift was reduced. A correlation analysis showed that the number of fiber bundles per unit volume was positively correlated with matrix modulus, strength, and dynamic moduli, though this parameter was negatively correlated with phase shift. Fiber diameter also was negatively correlated with scaffold strength. This study demonstrates how microstructural features can be related to the mechanical function of protein matrices and provides insight into structure-function relationships in such materials. This information can be used to identify and promote desirable microstructural features when designing biomaterials and engineered tissues.