An ultrasonic instrument includes a body, an actuation assembly, a shaft assembly, an ultrasonic blade, and a mechanical lockout. The body is configured to receive an ultrasonic transducer. The actuation assembly includes an activation lever that is configured to move from a first activation position toward the longitudinal axis to a second activation position. The activation lever is oriented obliquely relative the longitudinal axis in the second activation position. The shaft assembly includes an acoustic waveguide. The ultrasonic blade is in acoustic communication with the acoustic waveguide. The activation lever is operable to trigger ultrasonic activation of the ultrasonic blade by moving to the second activation position. The mechanical lockout is operable to selectively restrict movement of the first activation lever to the second activation position.