We report the discovery and Doppler mass measurement of a 7.4days 2.3R{Earth} mini-Neptune around a metal-poor K-dwarf BD+29-2654 (TOI-2018). Based on a high-resolution Keck/HIRES spectrum, the Gaia parallax, and multiwavelength photometry from the UV to the mid-infrared, we found that the host star has Teff=4174_-42_^+34^K, logg=4.62_-0.03_^+0.02^, [Fe/H]=-0.58{+/-}0.18, M=0.57{+/-}0.02M{sun}, and R=0.62{+/-}0.01R{sun}. Precise Doppler measurements with Keck/HIRES revealed a planetary mass of Mp=9.2{+/-}2.1M{Earth} for TOI-2018b. TOI-2018b has a mass and radius that are consistent with an Earthlike core, with a ~1%-by-mass hydrogen/helium envelope or an ice-rock mixture. The mass of TOI-2018b is close to the threshold for runaway accretion and hence giant planet formation. Such a threshold is predicted to be around 10M{Earth} or lower for a low-metallicity (low-opacity) environment. If TOI-2018b is a planetary core that failed to undergo runaway accretion, it may underline the reason why giant planets are rare around low- metallicity host stars (one possibility is their shorter disk lifetimes). With a K-band magnitude of 7.1, TOI-2018b may be a suitable target for transmission spectroscopy with the James Webb Space Telescope. The system is also amenable to metastable Helium observation; the detection of a Helium exosphere would help distinguish between a H/He-enveloped planet and a water world.