One of the most powerful figures in the Australian media sector, Harold Mitchell, is going global. Yesterday, London-based media group Aegis announced it would pay acquire Mitchell’s media buying and communications firm Mitchell Communications Group in a $363 million cash-and-shares deal.

Mitchell, who owns a 30% stake in Mitchell Communications (his immediate family owns another 10%), will get $120 million worth of shares in Aegis, making him the second largest shareholder in the global company.

Mitchell will stay on at the chairman of Aegis’ enlarged Asia Pacific business and appears likely to take a seat on the company’s board.

Based on the acquisition price of $1.20 a share, Mitchell’s total fortune should rise to about $280 million.

It’s an impressive jump, given BRW valued Mitchell’s wealth at $180 million in May 2009. In just over 12 months, Mitchell has made about $100 million.

Mitchell is clearly excited about the prospect of going global. As he told the Herald Sun: “Our Australian dollar is at a 20-year high against the English pound and it’s an opportunity to convert into a company which is exactly like us — an independent, entirely media company.”

There is little doubt the thing that made Mitchell Communications so attractive to Aegis was the company’s dominant position in the Australian market. Since establishing the company in 1976, Harold Mitchell has grown his business to the point where it books an estimated 10% of all media advertising in Australia. It has $900 million in media billings each year, an impressing $210 million ahead of its nearest rival. That sort of dominant market position has made Mitchell very powerful and his company very valuable.

Another key to Mitchell’s wealth creation is diversification, inside Mitchell Communications (where there are 21 separate businesses) and outside, where his wealth is spread across property, agribusiness (he owns a stake in a Western Australian cattle station) and even sporting teams (he part-owns Melbourne’s new rugby union team, the Melbourne Rebels).

Mitchell was quick to seize on the growth in digital advertising. He was one of the first movers into this sector in 1999 and more importantly was able to come out of the other side of the dotcom crash. Mitchell Communication’s projections estimate digital advertising will grow by more than 50% over the next three-to-four years, to the point where digital will account for more than 25% of the entire ad market. Given margins in digital are better than in traditional media, Mitchell’s perseverance in this market will pay off.

This is a renowned negotiator; he’s overseen a string of deals in the past few years as Mitchell Communications has expanded geographically and into new areas of the media industry, such as PR. Perhaps his best deal, however, was selling Mitchell Communications to listed digital agency eMitch — which Mitchell himself founded in 1999 and was a 30% shareholder — in late 2006 in a $100 million deal. He and his family took $66 million in cash from the deal, and used it as a springboard for growth.

And the latest money-making strategy could be his smartest. While his company’s operators are concentrated on Australia, the fact he has well-established connections with major global companies — such as SingTel, Simplot and BMW — and the backing of a global communications firm in Aegis could allow him to grow in a way not previously possible.

It is also worth noting that Mitchell now has direct exposure to European advertising markets. This could be very good or very worrying, depending on how the European economy recovers and how European advertising markets react to this.