The 55-year-old was born in Glasgow and attended the city's Notre Dame school.
She has previously worked in the publishing industry, where she helped launch in-house magazines for Asda and Waitrose - as well as working for Newsquest Herald & Times Group - before studying for a law degree in 2002.
Ms Mechan's appointment was confirmed after no other candidate stood against her.
Acting general secretary Alan McKenzie said: "We are thrilled to welcome Sheila Mechan to the SSTA, bringing with her a huge range of skills necessary for the post.
"We stand on the edge of hugely important development in Scottish education and our members may be assured that the SSTA is in the hands of a highly-skilled general secretary. We wish her well in her endeavours."