Implementations of a modular scope mount assembly are provided. In some implementations, the modular scope mount assembly may be used to secure a telescopic sight to a firearm (e.g., a rifle and/or a carbine). In some implementations, the modular scope mount assembly may be used to co-mount a telescopic sight and a reflex sight to a firearm. In some implementations, the modular scope mount assembly may be configured to place a reflex sight on the right side and/or left side of a co-mounted telescopic sight. In some implementations, the modular scope mount assembly may be configured to allow a user to change (increase or decrease) the eye relief between the user and the co-mounted reflex sight(s).