Sealevel Systems was founded by Tom O’Hanlan and his wife Susan, in 1986.

In 1991 the company released a dual port serial card that allowed users to set its I/O addresses to any two COM ports. In 1994, Sealevel developed the RS-485 auto-enabled circuit. The circuit eliminated the need to control the RS-485 transceiver-enable signal via software and removed the risk of communications error due to bus communications.

In 2005, Sealevel Systems released the industry’s first RoHS-compliant serial I/O board. In 2008, Sealevel won a defense contract for a USB/serial port cable with a heavily encased circuit board. The cable allows soldiers in the field to link laptops to AN/PRC-117F Multiband Manpack Radio (MBMMR) tactical radios, manufactured by any company, and transmit data, including GPS maps, images, coordinates and IM-type communications via radio signal instead of by satellite. It took seven years for the company to perfect the technology. In 2013 the company was awarded a sole-source contract for Naval Air Systems Command for this cable.