A Human, he was widely respected and hailed as a visionary, his area of expertise being time. He was a professor teaching at a university in Paris in 2342. Despite his brilliance, his theories on time and gravity did not gain widespread acceptance, with the result that in 2349 he left Earth with a team of fellow scientists in an attempt to test his theories.
After searching for two years for a suitable site, the team settled on Vandor IV, where they built two labs. The dense gravity of the pulsar in that system was required for the time-gravity experiments he needed to carry out. Manheim believed there were infinite dimensions, and that if the linearity of time could be changed, then a "window" to these dimensions could be opened up. As he came closer to proving his theories, he installed a security system in the lab, as well as a force field. He also made his wife stay in a protected room whenever he performed a new experiment.
He finally achieved his goal in 2364, when he opened a "window" into another dimension. However, this was at a serious cost to his health, as his mind was trapped between the two dimensions, and caused the destruction of one of the labs. He issued a distress call, which was answered by the USS Enterprise-D. Manheim himself was dying, and the temporal distortions caused by the open "window", or the "Manheim Effect", was spreading through the galaxy. The window had to be closed to prevent the other dimension ripping the fabric of the galaxy. This was achieved with the help of Lieutenant Commander Data, causing Manheim to recover completely. Although unable to adequately describe the other dimension, he remained on Vandor with his wife to continue his work, saying he owed it to those that died in the other lab. (TNG: "We'll Always Have Paris")
In 2365, when the Enterprise encountered a duplicate version of Captain Picard, apparently transported six hours back in time, the contemporary Captain Picard rejected Manheim's experiments with gravity and time as a cause of this anomaly - noting that the experiments were rudimentary and uncontrollable. (TNG: "Time Squared")