The objectives were to compare differences in telomere length (TL) among younger (21–54 years) and older adults (≥55) with mild traumatic brain injury (mTBI) to non-injured controls and to examine the association between TL and the severity of post-concussive symptoms over time. We performed a quantitative polymerase chain reaction to determine the TL (Kb/genome) of peripheral blood mononuclear cell samples (day 0, 3 months, and 6 months) from 31 subjects. The Rivermead Post-Concussion Symptoms Questionnaire was used to assess symptoms. Group-by-time comparisons of TL and symptom severity were evaluated with repeated-measures analysis of variance. Multiple linear regression examined the relationship between TL, group (mTBI and non-injured controls), and symptom severity total and subscale scores. Significant aging-related differences in TL were found within mTBI groups by time (day 0, 3 months, and 6 months;p= 0.025). Older adults with mTBI experienced significant worsening of changes in total symptom severity scores over time (day 0, 3 months, and 6 months;p= 0.016). Shorter TLs were associated with higher total symptom burden among each of the four groups at day 0 (baseline;p= 0.035) and 3 months (p= 0.038). Shorter TL was also associated with higher cognitive symptom burden among the four groups at day 0 (p= 0.008) and 3 months (p= 0.008). Shorter TL was associated with higher post-injury symptom burden to 3 months in both older and younger persons with mTBI. Large-scale, longitudinal studies of factors associated with TL may be useful to delineate the mechanistic underpinnings of higher symptom burden in adults with mTBI.