  Nitrogen-vacancy (NV) centers have considerable promise as high sensitivity
magnetometers, however are commonly limited by inefficient collection and low
contrasts. Laser threshold magnetometry (LTM) enables efficient collection and
high contrasts, providing a path towards higher sensitivity magnetometry. We
demonstrate an infrared LTM using an ensemble of NV centers in a single crystal
diamond plate integrated into a vertical external cavity surface emitting
laser. The laser was tuned to the spin dependent absorption line of the NV
centers, allowing for optical readout by monitoring the laser output power. We
demonstrate a magnetic sensitivity of 7.5~nT/$\sqrt{\textit{Hz}}$ in the
frequency range between 10 and 50 Hz. Furthermore, the contrast and the
projected PSNL sensitivity are shown to improve significantly by operating
close to the lasing threshold, achieving 18.4\% and
26.6~pT/$\sqrt{\textit{Hz}}$ near threshold. What's more, an unexpected
saturable absorption phenomenon was observed near threshold, which enhanced the
contrast and projected PSNL sensitivity.
