We present estimates of ultimate tensile strength (UTS) for two engineered β-solenoid protein mutant fibril structures (spruce budworm and Rhagium inquisitor antifreeze proteins) derived from sonication-based measurements and from force pulling molecular dynamics simulations, both in water. Sonication experiments generate limiting scissioned fibrils with a well-defined length-to-width correlation for the mutant spruce budworm protein and the resultant UTS estimate is 0.66 0.08 GPa. For fibrils formed from engineered R. inquisitor antifreeze protein, depending upon geometry, we estimate UTSs of 3.5 3.2–5.5 5.1 GPa for proteins with interfacial disulfide bonds, and 1.6 1.5–2.5 2.3 GPa for the reduced form. The large error bars for the R. inquisitor structures are intrinsic to the broad distribution of limiting scission lengths. Simulations provide pulling velocity-dependent UTSs increasing from 0.2 to 1 GPa in the available speed range, and 1.5 GPa extrapolated to the speeds expected in the sonication experiments. Simulations yield low-velocity values for the Young's modulus of 6.0 GPa. Without protein optimization, these mechanical parameters are similar to those of spider silk and Kevlar, but in contrast to spider silk, these proteins have a precisely known sequence-structure relationship.