We present a first strong-lensing model for the galaxy cluster RM J121218.5+273255.1 (z = 0.35; hereafter RMJ1212; also known as A1489). This cluster is among the top 0.1% richest clusters in the redMaPPer catalog; it is significantly detected in X-ray and through the Sunyaev-Zel'dovich effect in ROSAT and Planck data, respectively, and its optical luminosity distribution implies a very large lens, following mass-to-light scaling relations. Based on these properties it was chosen for the Webb Medium Deep Fields (WMDF) James Webb Space Telescope/Guaranteed Time Observations program. In preparation for this program, RMJ1212 was recently imaged with Gemini Multi-Object Spectrograph (GMOS) on Gemini North and in seven optical and near-infrared bands with the Hubble Space Telescope (HST). We use these data to map the inner mass distribution of the cluster, uncovering various sets of multiple images. We also search for high-redshift candidates in the data, as well as for transient sources. We find over a dozen high-redshift (z ⪆ 6) candidates based on both photometric redshift and the dropout technique. No prominent (⪆5σ) transients were found in the data between the two HST visits. Our lensing analysis reveals a relatively large lens with an effective Einstein radius of θ E ≃ 32 3″ (z s = 2), in broad agreement with the scaling-relation expectations. RMJ1212 demonstrates that powerful lensing clusters can be selected in a robust and automated way following the light-traces-mass assumption.