Linear morphea is the most common subtype of localized scleroderma in the pediatric population. This condition can be quite disabling, with complications such as growth defects and painful flexion contractures. Assessment of disease progression and early intervention are key to minimize morbidity. We report linear morphea in a previously healthy 12-year-old girl. The patient presented with a one-year history of a linear plaque crossing her left antecubital fossa, measuring 7x3cm. The diagnosis was confirmed by biopsy, in which deep tissue involvement was noted. Subsequent management and evaluation of the disease activity was done by ultrasound, which allowed precise guidance of pharmacotherapy. The patient improved both clinically and sonographically with a methotrexate course. Sonographic changes accurately described the disease activity on follow up assessments. Features suggestive of an active phase include a thickened and hypoechoic dermis contrasting hyperechoic subcutaneous tissue. The atrophic stage is characterized by a thinned-out dermis and subcutaneous area. Typical vascular traits of each disease phase can also contribute to the assessment. Ultrasound is a grossly underused tool in the field of dermatology. It can provide accurate and sensitive information about disease activity in linear morphea, allowing for more timely intervention and optimal patient management.