Sega did not form any direct distribution channels in Europe until the mid-1990s, so the Mega Drive's launch in Europe was somewhat disorganized. For one, the PAL Mega Drive was delayed - originally set to be released in 1989, it was pushed back to March 1990 due to manufacturing issues, and did not reach consumers until later in the year. By the time the Mega Drive hit places like Spain, for example, it was almost on its last leg in Japan. The late release saw a handful of games, primarily those released in 1990 in Japan or North America (most notably games released by Renovation Products), skip the European Mega Drive altogether, although this would be made up for in later years by more local releases.
Europe itself was not treated as one region until the late 1990s, so each country received games and hardware at different points in time (although only weeks and months, as opposed to years, separated launches). In the United Kingdom, the Mega Drive launched in September 1990 during the European Consumer Electronics Show (ECES) for the price of £189.99 (complete with Altered Beast), however the delay meant that many keen gamers had imported systems prior to this date (and magazines were reviewing imported games).
Virgin Mastertronic distributed the console in the UK, as they had with the Master System though also extended their reach to France and Germany. Sega bought the Mastertronic side of the company in September 1991 and began distributing consoles and software themselves, thus establishing Sega Europe.
Success of the Mega Drive was initially somewhat hard to measure, not least because in much of Europe, consoles were not particularly popular. Whereas Japan and North America had opted for dedicated video game consoles during the 1980s, the rest of the world was content with home computers, of which 16-bit varieties (the Commodore Amiga and Atari ST) had been launched around 1985-1987. Many consumers therefore saw no need for the Mega Drive - it was more expensive to adopt, yet less functional than a 16-bit computer, and was thus seen as a novelty item for wealthier families or something engineered for kids.
Though Nintendo had had a similar problem with the NES (and would continue to do so with the SNES, released very late in 1992), Sega reversed their fortunes with the release of Sonic the Hedgehog, on the same date as the US, June 23, 1991 (though some magazines thought it had a July release). This attracted numerous big software houses (such as Virgin Games, Acclaim Entertainment, and Ocean Software) and building a strong portfolio of games. Much of this success was built on the Sega Master System, which although had faced huge competition from computers, had a respectable install base and was the most successful home console in much of Europe.
Like North America, the Mega Drive was replaced, perhaps prematurely, by the Sega Saturn in 1995 and discontinued in 1998 after 8 years in the European market. However, during its half-decade of service had become not only the most successful console of all time, but had began to change opinions on gaming as a whole. The IBM PC was putting traditional 16-bit computers out of action, with mice and keyboards being favored over joysticks - games built for joysticks, which had once thrived on machines by Atari, Sinclair, Amstrad and Commodore found themselves on consoles instead. The Mega Drive also helped launch the hugely popular FIFA series, which continues to exist to this day.
The Mega Drive was the system of choice in the United Kingdom (there, it was known as the #1 retro games console of all time), and likely extended its dominance to France, Germany, Spain and Portugal. It was also big in Italy, Greece and Scandinavian countries, although much of the story is undocumented. Australia, though not in Europe, relied much on European stock, with the Mega Drive being a relatively successful console there.