Bain's decision to step down as CEO follows a period of serious ill-health earlier this year, from which she has now fully recovered.
In a statement announcing her exit, the widely-respected exec said she had "endured a difficult year, overcoming breast cancer and other related conditions."
"I have now officially received the all-clear and am feeling fighting fit," said Bain, "but after experiencing a life-threatening condition and reviewing my priorities I feel the time is right to hand-over the reins to someone else, and focus on other ambitions, of which I have many."
She added that it has been "a great honour" to serve BASCA and wished the organization and its members "the very best for the future."
During her tenure as CEO, Bain led BASCA through an extensive modernization programme, partnering with universities across the United Kingdom and consistently campaigning for greater inclusivity and diversity within the music business, as well as a fair share of digital revenues for songwriters.
She also reinvigorated The BASCA Trust, which had lain dormant for almost two decades, bringing in new trustees and partnering with the PRS Foundation to provide bursaries for homegrown songwriters and composers, alongside education events.
Earlier this year, BASCA launched its #soldforasong campaign, calling for music publishing companies to share the spoils of licensing agreements agreed with Facebook, as well any "financial windfall" resulting from Spotify's public listing.
More recently, the organization spoke out against Sony's acquisition of EMI Music Publishing, warning that the Japan-based company would become a "major superpower" in the music industry, dominating licensing markets and limiting creator choice as a result of the deal.
In recognition of her achievements, Bain was inducted onto Music Week's Women in Music Roll of Honour and placed 29th in BBC Radio 4's 2018 Woman's Hour 'Power List.'