Aim: The 40-Something Randomised Controlled Trial (RCT) aimed to determine if a 12-month health professional-led intervention could improve dietary intake and increase physical activity for weight gain prevention in non-obese premenopausal women. Methods: Fifty-four women were randomised to a motivational interviewing intervention (MI) (n = 28; five health professional consultations) or a self-directed intervention (SDI; n = 26; written advice). Weight control behaviours including dietary intake (four-day weighed food record) and physical activity (pedometer steps; minutes spent in light, moderate and vigorous activity; sitting time) were measured at baseline, and at 3 and 12 months. Participants completed a questionnaire containing demographic questions, Short Form 36 and the International Physical Activity Questionnaire at baseline and 12 months. Linear mixed model analyses adjusting for baseline and 3-month behaviour, covariates and interactions were used to test for between-group differences. Results: At 12 months, the MI group had diets significantly more nutrient dense in iron (P = 0.01) and potassium (P = 0.04), and consumed more fruits (P = 0.02) than the SDI women. Women who achieved their weight control goals consumed significantly more daily fruit servings (+0.76 servings/day, P = 0.02) and less meat/meat alternatives (−0.34 servings/day, P < 0.01) than women who did not achieve weight control. The lack of changes in physical activity indicates that more emphasis may need to be placed on promoting physical activity. Conclusions: The findings demonstrated the effectiveness of the 40-Something RCT in improving iron and potassium nutrient density and fruit servings per day in women soon to commence menopause.