Yes. Man was immortal before sin, because a thing may be incorruptible on the part of its efficient cause.
Ante peccatum homo erat immortalis, quia dicitur aliquid incorruptibile ex parte causae efficientis.
For man's body was indissoluble not by reason of any intrinsic vigor of immortality, but by reason of a supernatural force given by God to the soul, whereby it was enabled to preserve the body from all corruption so long as it remained itself subject to God.
Non enim corpus eius erat indissolubile per aliquem immortalitatis vigorem in eo existentem; sed inerat animae vis quaedam supernaturaliter divinitus data, per quam poterat corpus ab omni corruptione praeservare, quandiu ipsa Deo subiecta mansisset.
This power of preserving the body was not natural to the soul, but was the gift of grace. And though man recovered grace as regards remission of guilt and the merit of glory; yet he did not recover immortality, the loss of which was an effect of sin; for this was reserved for Christ to accomplish, by Whom the defect of nature was to be restored into something better, as we shall explain further on (III, Q14, A4, ad 1).
Vis illa praeservandi corpus a corruptione, non erat animae humanae naturalis, sed per donum gratiae. Et quamvis gratiam recuperaverit ad remissionem culpae et meritum gloriae, non tamen ad amissae immortalitatis effectum. Hoc enim reservabatur Christo, per quem naturae defectus in melius reparandus erat, ut infra dicetur.
The promised reward of the immortality of glory differs from the immortality which was bestowed on man in the state of innocence.
Differt immortalitas gloriae, quae promittitur in praemium, ab immortalitate quae fuit homini collata in statu innocentiae.
It is written (Romans 5:12): "By sin death came into the world."
Dicitur ad Rom. V, quod "per peccatum intravit mors in mundum".