Long-period transiting exoplanets bridge the gap between the bulk of transit- and Doppler-based exoplanet discoveries, providing key insights into the formation and evolution of planetary systems. The wider separation between these planets and their host stars results in the exoplanets typically experiencing less radiation from their host stars; hence, they should maintain more of their original atmospheres, which can be probed during transit via transmission spectroscopy. Although the known population of long-period transiting exoplanets is relatively sparse, surveys performed by the Transiting Exoplanet Survey Satellite (TESS) and the Next Generation Transit Survey (NGTS) are now discovering new exoplanets to fill in this crucial region of the exoplanetary parameter space. This study aims to characterise a new long-period transiting exoplanet by following up on a single-transit candidate found in the TESS mission. The TOI-4862 system was monitored using a combination of photometric instruments (TESS, NGTS, and EulerCam) and spectroscopic instruments (CORALIE, FEROS, HARPS, and PFS) in order to determine the period, radius, and mass of the long-period transiting exoplanet NGTS-30 b/TOI-4862 b. These observations were then fitted simultaneously to determine precise values for the properties and orbital parameters of the exoplanet, as well as the refined stellar parameters of the host star. We present the discovery of a long-period (P=98.29838+/-0.00010day) Jupiter-sized (0.928+/-0.032R_J_; 0.960+/-0.056M_J_) planet transiting a 1.1Gyr old G-type star, one of the youngest warm Jupiters discovered to date. NGTS-30 b/TOI-4862 b has a moderate eccentricity (0.294_-0.010_^0.014^), meaning that its equilibrium temperature can be expected to vary from 274_-46_^+30^K to 500_-84_^+55^K over the course of its orbit. Through interior modelling, NGTS-30 b/TOI-4862b was found to have a heavy element mass fraction of 0.23_-0.06_^+0.05^ and a heavy element enrichment (Zp/Z*) of 20_-6_^+5^, making it metal-enriched compared to its host star. NGTS-30 b/TOI-4862 b is one of the youngest well-characterised long-period exoplanets found to date and will therefore be important in the quest to understanding the formation and evolution of exoplanets across the full range of orbital separations and ages.