Pekka Lehtinen started his career as a malacologist; his MSc thesis dealt with terrestrial gastropods in the Finnish archipelago.

From the time Lehtinen started to work on his PhD thesis up until the present, he has visited almost all the leading museums in the world (he visited the MNHN in Paris 11 times) and/or loaned types and comparative material in order to study poorly available faunas collected from remote islands and territories.

After gaining a permanent position at the University of Turku, Lehtinen started to carry out extensive expeditions world-wide, a practice which he has continued to the present.

Since 1969 he has visited over 70 countries in search for spiders, mites, harvestmen, pseudoscorpions, terrestrial isopods, myriapods, molluscs and other invertebrates.

Some of the countries he visited four or even more times, such as India, Philippines, Sri Lanka, Thailand, Malaysia, Indonesia and French Polynesia.

Pekka Lehtinen has participated in all international congresses of arachnology: the pre-CIDA Congress in Frankfurt am Main (1965), and then in Paris (1968), Brno (1971), Amsterdam (1974), Exeter, UK (1977, as an invited speaker), Vienna (1980), Panama (1983), Jaca, Spain (1986), Turku (1989), Brisbane (1992), Geneva (1995), Chicago (1998), Badplaas, South Africa (2001, as an invited speaker), Ghent, Belgium (2003), Saõ Pedro, Brazil (2007), Siedlce, Poland (2010) and Kenting, Taiwan (2013).

Lehtinen has always been active in nature conservation projects, both in field studies and (from 1983) as a member of different committees and working groups, for example, in compiling the Finnish Red Data Books.

Lehtinen was active on the world forum several times, for example, as a member of the Organizing Committee of the ICSEB (International Congress of Systematic and Evolutionary Biology) held in 1990 in Washington, D.C., and in 1996 in Budapest, when he served as a member of the IOSEB Council (International Organization for Systematic and Evolutionary Biology).