<PAGE>

                                                                    Exhibit 10.5

                                                                  EXECUTION COPY

                            [Platinum US Letterhead]

                                                                   June 24, 2004

Ms. H. Elizabeth Mitchell
107 West 89th Street
Apt. 2B
New York, New York 10024

Dear Liz:

            Reference is made to the letter agreement dated October 14, 2002,
from St. Paul Re, Inc. to you ( the "First Letter Agreement"), which has been
assigned to Platinum Underwriters Reinsurance, Inc., a Maryland corporation
("Platinum"). I am writing this letter (the "Letter Agreement") to confirm our
understanding regarding the additional benefits to which you will be entitled
upon your separation from employment with Platinum. This Letter Agreement is
intended to supplement the First Letter Agreement.

            1. Termination of Employment.

            (a)   Termination for Good Reason or Without Cause. If you terminate
your employment for "Good Reason" (as defined below) or if your employment is
terminated by Platinum without "Cause," (as defined below) you will receive,
immediately upon the effectiveness of any such termination, a lump sum cash
payment equal to the sum of (i) one year's base salary and target bonus, and
(ii) any earned but unpaid base salary or other amounts (including reimbursable
expenses and any vested amounts or benefits under Platinum's otherwise
applicable employee benefit plans or programs) accrued and owing through the
date of termination. The foregoing payment will be conditioned upon you
executing and honoring a standard waiver and release of claims in favor of
Platinum in a form determined by Platinum.

      (b)   Termination Other than for Good Reason; Termination for Cause. If
you terminate your employment other than for Good Reason or if your employment
is terminated by Platinum for Cause, you will receive no further payments,
compensation or benefits under this Letter Agreement, except you will be
eligible to receive, upon the effectiveness of such termination, amounts
(including reimbursable expenses and any vested amounts or benefits under
Platinum's employee benefit plans or programs) accrued or owing prior to the
effectiveness of such termination.

            (c)   Death or Disability. Upon the termination of your employment
on account of your death or Disability, you or your beneficiaries will receive
(i) any unpaid base salary through the date of termination plus a pro-rata
portion through the date of termination of your target bonus for the year of
termination, and (ii) all other unpaid amounts (including reimbursable expenses
and any vested amounts or benefits under Platinum's employee benefit plans or
programs) accrued or owing prior to the effectiveness of such termination.

                                       1
<PAGE>

                                                                  EXECUTION COPY

   (d)   Definitions.

      (i)   Cause. For purposes of this Letter Agreement, "Cause" means (i) your
willful and continued failure to substantially perform your duties as Executive
Vice President and Chief Operating Officer of Platinum; (ii) your conviction of,
or plea of guilty or nolo contendere to, a felony or other crime involving moral
turpitude; or (iii) your engagement in any malfeasance or fraud or dishonesty of
a substantial nature in connection with your position with Platinum or its
subsidiaries, or other willful act that materially damages the reputation of
Platinum or its subsidiaries.

      (ii)  Good Reason. For purposes of this Letter Agreement, "Good Reason"
means (i) Platinum reduces your base salary or your target bonus without your
express written consent; (ii) Platinum reduces the scope of your duties,
responsibilities or authority without your express written consent; (iii)
Platinum requires you to report to anyone other than the President; (iv)
Platinum requires you to be principally based other than in Platinum's offices
in New York; or (v) Platinum breaches any other material provision of this
Letter Agreement or the First Letter Agreement; provided, however, that if you
voluntarily consent to any reduction or change described above in lieu of
exercising your right to resign for Good Reason and deliver such consent to
Platinum in writing, then such reduction, transfer or change shall not
constitute "Good Reason" hereunder, but you shall have the right to resign for
Good Reason under this Letter Agreement as a result of any subsequent reduction
described above.

      (iii) Disability. For purposes of this Letter Agreement, "Disability"
means a termination of your employment by Platinum if you have been rendered
incapable of performing your duties to Platinum by reason of any medically
determined physical or mental impairment that can be expected to result in death
or that can be expected to last for a period of either (i) six or more
consecutive months from the first date of your absence due to the disability or
(ii) nine or more months during any twelve-month period.

            2.    Miscellaneous Provisions.

            (a)   This Letter Agreement may not be amended or terminated without
the prior written consent of you and Platinum. This Letter Agreement will be
binding on and inure to the benefit of our respective successors and, in your
case, your heirs and other legal representatives. Other than as provided herein,
the rights and obligations described in this Letter Agreement may not be
assigned by either party without the prior written consent of the other party.

            (b)   This Letter Agreement will be governed by and construed and
enforced in accordance with the laws of the State of New York without reference
to rules relating to conflict of laws. All disputes arising under or related to
this Letter Agreement will be settled by arbitration under the Commercial
Arbitration Rules of the American Arbitration Association then in effect as the
sole and exclusive remedy of either party. Such arbitration shall be held in New
York City. Any judgment on the award rendered by such arbitration may be entered
in any court having jurisdiction over such matters. Each party's costs and
expenses of such arbitration, including reasonable attorney fees and expenses,
shall be borne by such party, unless you are the

                                       2
<PAGE>


                                                                  EXECUTION COPY

prevailing party in the award entered in such arbitration, in which case, all
such costs and expenses shall be borne by Platinum.

            (c)   This Letter Agreement supercedes any inconsistent provisions
of any plan or arrangement regarding severance payments that would otherwise be
applicable to you to the extent such provisions would limit any rights granted
to you hereunder or expand any restrictions imposed on you hereby.

            This Letter Agreement is intended to be a binding obligation upon
Platinum and yourself. If this Letter Agreement correctly reflects your
understanding, please sign and return one copy to me for Platinum's records.

                                      PLATINUM UNDERWRITERS REINSURANCE, INC.

                                      By: /s/ Michael D. Price
                                          -----------------------------------
                                          Michael D. Price
                                          President

The above Letter Agreement correctly reflects our understanding, and I hereby
confirm my agreement to the same.

/s/ H. Elizabeth Mitchell
------------------------------------
H. Elizabeth Mitchell

Dated as of June 24, 2004

                                       3