His collections featured designs inspired by ancient Greek vases,[4] Byzantine manuscripts, traditional Greek wood carvings and paintings; African,[5] Chinese,[6] Indonesian, Russian,[7] and Spanish art; heraldry;[8] insects;[9] cartoons; Persian tapestries;[10] the Unicorn tapestries; and paintings by the Impressionists,[11][12] Dominicos Theotokopoulos (El Greco),[3]: 159–160  Henri Rousseau,[13] and Yannis Gaitis.

[3]: 160–161 By the late 1970s and 1980s the Tseklenis brand name could also be found on distinguished lines of house linens, wall and floor tiles, luggage, hosiery, uniforms, as well as the interior designs of cars, aircraft and hotels.

Most recently, he was particularly active in the interior and exterior design and refurbishment of hotels, luxury housing estates, and public transport vehicles.

[17] At the age of 15 Yannis Tseklenis started working at his father's textile-couture retail business, where he gained valuable experience while also exploring his abilities in painting and design.

[22] Bernadine Morris wrote in The New York Times that "...many of the clothes achieve their distinction from Mr. Tseklenis' prints, which look like modern abstractions but are derived from such ancient sources as a Minoan octopus and a Corinthian vase..."[14] This motivated him to set up his own fashion business.

[25] During the 1970s Tseklenis became the force behind the organisation of the first fashion collections in Greece, also using his international influence to promote Greek designers abroad.

[26] In 1970 he licensed his ladies' dresses to Frank Usher (UK) and to Madison S.A (Greece),[27] while in 1971 he opened Tseklenis Boutiques in Beirut, Kuwait and Riyadh, as well as showrooms in Osaka (Japan) via the firm Misaki Sojhi.

By 1973 Tseklenis had permanent showrooms in London and New York, thus supplying most of the top UK and US fashion stores of the time.

While suffering an aggressive form of melanoma, he was admitted to the Memorial Hospital in New York, where doctors amputated his left arm to prevent the spread of cancer.

He thus established Tseklenis International Fashion Enterprises (ladieswear, menswear, accessories, bodywear, action-sportwear and linens) based in Athens, in collaboration with Minion department stores.

The decade found him expanding his business even further and entering into activities such as interior design, bed linen, homewares (ceramics, tableware, candlesticks, etc.