I get asked this question frequently by people who are considering a career change. Here is my story.
The first 10 formative years of my career were spent with McKinsey, a strategy consulting firm, working in the London and Amsterdam offices with projects pretty much in every country in Europe. My stay there was a bit different than the norm: 1) I stuck around for about 10 years, versus 2-3 on average, and 2) I did not specialise in one industry (banking, consumer goods, etc.), but rather did a functional specialisation: mergers and acquisitions, usually on the sell side. In short, I helped big Fortune 500 companies sell themselves to other big Fortune 500 companies. As a result, every project that I worked on was in a different industry, and I had to adjust rapidly to understand a completely new field of work (beer, pet food, DIY retailing, petrol retailing, grocery, retail banking, asset management, insurance, private equity, e-commerce, mobile payments, postal services and logistics just to name a few). Little did I know then, how useful this skill would come in a decade later in a briefing for a new presentation design project.
After leaving the Firm, I moved with my family to Tel Aviv where I started out as an independent strategy consultant. Soon, I got in touch with the Israeli high tech industry. The small startups could not afford (and probably did not need) me as a strategy consultant for 6 months but saw value in my PowerPoint slides for meetings with potential investors (these charts were still B&W, highly organised, full of consulting speak at that time). It was here that my gradual transformation to a presentation designer started. Gradual is important here, I think no freelancer figures out exactly his professional niche from day one. In my case it probably took around 2-3 years to stop calling myself a strategy consultant.
So, after the Israeli startups came the Israeli VCs, I started writing the blog, and in came international clients (most of my clients are in the US now), and increasingly the McKinsey-style big companies came back as clients, this time for presentation design.
So, here is the story. Two things to remember. Finding your niche takes time, and it depends highly on the coincidences that formed your specific skill set.
Good luck with your journey!